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PHP : Language Reference : Operators

Chapter6.Operators

An operator is something that you feed with one or more values (or expressions, in programming jargon) which yields another value (so that the construction itself becomes an expression). So you can think of functions or constructions that return a value (like print) as operators and those that return nothing (like echo) as any other thing.

There are three types of operators. Firstly there is the unary operator which operates on only one value, for example ! (the negation operator) or ++ (the increment operator). The second group are termed binary operators; this group contains most of the operators that PHP supports, and a list follows below in the section Operator Precedence.

The third group is the ternary operator: ?:. It should be used to select between two expressions depending on a third one, rather than to select two sentences or paths of execution. Surrounding ternary expressions with parentheses is a very good idea.

Operator Precedence

The precedence of an operator specifies how "tightly" it binds two expressions together. For example, in the expression 1 + 5 * 3, the answer is 16 and not 18 because the multiplication ("*") operator has a higher precedence than the addition ("+") operator. Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18. If operator precedence is equal, left to right associativity is used.

The following table lists the precedence of operators with the highest-precedence operators listed at the top of the table. Operators on the same line have equal precedence, in which case their associativity decides which order to evaluate them in.

Table6.1.Operator Precedence

Associativity Operators Additional Information
non-associative new new
left [ array()
non-associative ++ -- increment/decrement
non-associative ~ - (int) (float) (string) (array) (object) @ types
non-associative instanceof types
right ! logical
left * / % arithmetic
left + - . arithmetic and string
left << >> bitwise
non-associative < <= > >= comparison
non-associative == != === !== comparison
left & bitwise and references
left ^ bitwise
left | bitwise
left && logical
left || logical
left ? : ternary
right = += -= *= /= .= %= &= |= ^= <<= >>= assignment
left and logical
left xor logical
left or logical
left , many uses


Left associativity means that the expression is evaluated from left to right, right associativity means the opposite.

Example6.1.Associativity

<?php
$a
= 3 * 3 % 5; // (3 * 3) % 5 = 4
$a = true ? 0 : true ? 1 : 2; // (true ? 0 : true) ? 1 : 2 = 2

$a = 1;
$b = 2;
$a = $b += 3; // $a = ($b += 3) -> $a = 5, $b = 5
?>


Use parentheses to increase readability of the code.

Note:

Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.

Related Examples ( Source code ) » language.operators
















Code Examples / Notes » language.operators

11-jun-2004 03:22

Warhog wrote: "maybe usefull for some tricky coding and helpfull to prevent bugs :D"
I'm sure Warhog was being facetious, but for the new programmers in the audience I'd like to point out that 'tricky coding' and relying on precedence/order of evaluation are both well-known ways to *produce* bugs.
Use parentheses instead.


npeelman

Update to message by yasuo_ohgaki at hotmail dot com:
 I know this is an old message but when using an Associative array in a string you do not have to use { and  } to resolve ambiguity.
ex:
Associative Array in string:
$arr['item'] = 'string';
echo "This is {$arr['item']}"; //prints "This is string".
...does work but, so does:
echo "This is $arr[item]"; //prints "This is string".
... simply enclose the whole string with double quotes and leave out the single quotes from around the index name. This simplifies the code and makes things easier to read.


janturon

This is very common problem: set one variable to another, if it is not empty. If it is, set it to something else.
For example: set $bar to $foo, if $foo is empty, set $bar to "undefined";
if(!empty($foo)) $bar= $foo; else $bar= "undefined";
OR operator can shorten it:
$bar= @$foo or $bar= "undefined";


me

The scope resolution operator ::, which is missing from the list above, has higher precedence than [], and lower precedence than 'new'. This means that self::$array[$var] works as expected.

stopping

The low precedence of the OR operator is useful for error control statements such as this one:
$my_file = @file ('non_existent_file') or die ("Failed opening file: error was '$php_errormsg'");
Notice the good readability of the code.


kit dot lester

Table 15-1 omits the precedence of instanceof - testing suggests it to be of the same precedence as not, negate, casting, and @.
The table also omits the precedence (and associativity) of the "execution operator" - but since that's a sort of quoting, I don't think it meaningfully has a precedence or associativity - they explain what is to happen where there's an open-endedness in the sense of missing brackets, and the quoting is a sort of bracket. (At least: because of the execution operator's double-ended closedness, I can't figure out any code where it would matter, so I can't test it.)


golotyuk

Simple POST and PRE incremnt sample:
<?php
$b = 5;
$a = ( ( ++$b ) > 5 ); // Pre-increment test
echo (int)$a;
$b = 5;
$a = ( ( $b++ ) > 5 ); // Post-increment test
echo (int)$a;
?>
This will output 10, because of the difference in post- and pre-increment operations


dlyons

Re: Rick on 2-Sep-2005.
Actually, the C equivalent of "$a[$c++]=$b[$c++];" has undefined behavior, and the increments are by no means guaranteed to happen after the assignment in C.  (Search for a discussion of C "sequence points" for details.)


yasuo_ohgaki

Other Language books' operator precedence section usually include "(" and ")" - with exception of a Perl book that I have. (In PHP "{" and "}" should also be considered also). However, PHP Manual is not listed "(" and ")" in precedence list. It looks like "(" and ")" has higher precedence as it should be.
Note: If you write following code, you would need "()" to get expected value.
$bar = true;
$str = "TEST". ($bar ? 'true' : 'false') ."TEST";
Without "(" and ")" you will get only "true" in $str.
(PHP4.0.4pl1/Apache DSO/Linux, PHP4.0.5RC1/Apache DSO/W2K Server)
It's due to precedence, probably.


10-jun-2004 01:58

of course this should be clear, but i think it has to be mentioned espacially:
AND is not the same like &&
for example:
<?php $a && $b || $c; ?>
is not the same like
<?php $a AND $b || $c; ?>
the first thing is
(a and b) or c
the second
a and (b or c)
'cause || has got a higher priority than and, but less than &&
of course, using always [ && and || ] or [ AND and OR ] would be okay, but than you should at least respect the following:
<?php $a = $b && $c; ?>
<?php $a = $b AND $c; ?>
the first code will set $a to the result of the comparison $b with $c, both have to be true, while the second code line will set $a like $b and THAN - after that - compare the success of this with the value of $c
maybe usefull for some tricky coding and helpfull to prevent bugs :D
greetz, Warhog


sm

Note the highly unfortunate difference from Java, which associates the trinary operator right-to-left.
---------------------------- source
function trinaryTest($foo){  // works as you think in Java, but not PHP
$bar = $foo > 20
? "greater than 20"
: $foo > 10
? "greater than 10"
: $foo > 5
? "greater than 5"
: "not worthy of consideration";
echo $foo." =>  ".$bar."\n";
}
echo "\n\n\n----trinaryTest\n\n";
trinaryTest(21);
trinaryTest(11);
trinaryTest(6);
trinaryTest(4);
function trinaryTestParens($foo){

$bar = $foo > 20
? "greater than 20"
: ($foo > 10
? "greater than 10"
: ($foo > 5
? "greater than 5"
: "not worthy of consideration"));
echo $foo." =>  ".$bar."\n";
}
echo "\n\n\n----trinaryTestParens\n\n";
trinaryTestParens(21);
trinaryTestParens(11);
trinaryTest(6);
trinaryTestParens(4);
---------------------------- output
----trinaryTest
21 =>  greater than 5
11 =>  greater than 5
6 =>  greater than 5
4 =>  not worthy of consideration
----trinaryTestParens
21 =>  greater than 20
11 =>  greater than 10
6 =>  greater than 5
4 =>  not worthy of consideration


phpnet dot 20 dot dpnsubs

Note that in php the ternary operator ?: has a left associativity unlike in C and C++ where it has right associativity.
You cannot write code like this (as you may have accustomed to in C/C++):
<?
$a = 2;
echo (
   $a == 1 ? 'one' :
   $a == 2 ? 'two' :
   $a == 3 ? 'three' :
   $a == 4 ? 'four' : 'other');
echo "\n";
// prints 'four'
?>
You need to add brackets to get the results you want:
<?
$a = 2;
echo ($a == 1 ? 'one' :
       ($a == 2 ? 'two' :
       ($a == 3 ? 'three' :
       ($a == 4 ? 'four' : 'other') ) ) );
echo "\n";
//prints 'two'
?>


31-oct-2006 06:20

In response to:
----------------------
T Chan
19-Aug-2006 10:30
[...]
In response to the manual, associativity doesn't determine the order of evaluation - it determines how it's "bracketed": a=b=c=1 is equivalent to a=(b=(c=1)), but a is evaluated first (an important point if evaluating a has side-effects)
<?php
function one($str) {
  echo "$str";
  return 1;
}
$a = array(1,2);
$a[one("A")] = $a[one("B")] = 1; // outputs AB
?>
----------------------
The manual, stated "associativity determines order of evaluation," which is completely correct, as it uses a stack execution, as with C.
The example you provided to counter the manual's statement however, is contrived, as it uses function calls to output a variable which is not even used in assignment; this in turn disproves (in your opinion) the manual's statement. Fuction calls, have much higher precedence than the assignment operator.
However:
Right Associativity with the '=' operator means the following:
The expression:  $a = $b = 1;
IS indeed evaluated from right to left.
The stack is used as follows. (if you're not familiar with stacks / queues, look em up).
The expressions are pushed onto the stack in the following order:  
       $a     $b   =    1    =
       ^                         ^
      Bottom                 Top
The stack is then "evaluated" from top to bottom (which translates from right to left, if you're reading the expression)
It would read:  assign 1 to assign $b to $a
Which results in $a = 1
Try the following simple script:
<?php
function output(& $b)
{
  echo( "b value is: ".$b );
return $b;
}
$a = 2;
$b = 5;
echo ($a = $b = 1);
$a = 2;
$b = 5;
echo ( "a value is: ".$a = output($b) );
?>
Output is:
1
b value is: 5
a value is: 5
---------------------------------------
Your example had nothing to do with the assignment operator evaluation order, only procedural precedence.


edwardsbc

In response to npeelman at cfl dot rr dot com
29-Dec-2004 06:22:
You have misunderstood the behaviour of the interpreter.
With out the curly braces and the single quoted key identifier, the interpreter "assumes" you meant your CONSTANT to be a string.  This ONLY works within a parsed (double quoted) string.  And it doesn't help you at all if your array is multi-dimensional. I consider this a very bad habbit because it will get you in trouble elsewhere. Try the following:
<?php
define('item','AnyOldThing');
define('b',12);
$arr['item']['b'] = 'string';
$arr['AnyOldThing'][12]= 'Maybe not what I intended.';
echo "This is a {$arr['item']['b']}"; // [1] prints "This is a string".
echo "This is a $arr[item][b]"; // [2] broken
echo $arr[item][b]; // [3] broken
?>


madcoder

In response to mathiasrav at gmail dot com:
The reason for that behavior is the parentheses.  From the description:
"Parentheses may be used to force precedence, if necessary. For instance: (1 + 5) * 3 evaluates to 18."
So the order of operations says that even though the equality operator has higher precedence, the parentheses in your statement force the assignment operator to a higher precedence than the equality operator.
That said, it still doesn't work the way you expect it to.  Neither way works, for these reasons:
<?php
if ( $a != ($a = $b) )
?>
Order of operations says to do the parentheses first.  So you end up with:
<?php
$a = $b;
if ( $a != $a )
?>
Which is obviously going to be false.  Without the parentheses:
<?php
if ( $a != $a = $b )
?>
Order of operations says to do the inequality first, then the assignment, so you have:
<?php
if ( $a != $a );
$a = $b;
?>
Which again is not what you expected, and again will always be false.  But because you are only working with values of 0 and 1, you can make use of the XOR operator:
<?php
if ( $a ^= $b )
?>
This will only be true if 1) $a is 0 and $b is 1, or 2) $a is 1 and $b is 0.  That is precisely what you wanted, and it even does the assignment the way you expected it to.
<?php
foreach ($ourstring as $c) {
 if ($bold ^= $c['bold']) $resstring .= bold;
 if ($underline ^= $c['underline']) $resstring .= underline;
 $resstring .= $c[0];
}
?>
That code now works and produces the output you expected.


t chan

In response to 26-Mar-2001 08:53, parens don't need to have precedence. There's only one way to convert them to a syntax tree. I can't think of a sensible reason it could be ambiguous either (and sensible language designers will make ( always pair with ) so there can't be any ambiguity).
In response to 12-Aug-2005 08:47, you can do <?php $myvar OR ($myvar = 1); ?>, or the equivalent <?php !$myvar AND $myvar = 1; ?>, and if you have to use ifs, <?php if(!$myvar) { $myvar = 1; } ?> will do just fine (though in general, I'd be careful using boolean expressions as an indicator of success/failure when 0 could be a valid value. And you probably want to use isset($myvariable)).
In response to the manual, associativity doesn't determine the order of evaluation - it determines how it's "bracketed": a=b=c=1 is equivalent to a=(b=(c=1)), but a is evaluated first (an important point if evaluating a has side-effects)
<?php
function one($str) {
echo "$str";
return 1;
}
$a = array(1,2);
$a[one("A")] = $a[one("B")] = 1; // outputs AB
?>


22-aug-2004 05:51

I think warhog's note about the differing precedence between && / AND and || / OR is worth repeating.  Since && and || evaluate before the assignment operator (=) while AND and OR evaluate after it, you can get COMPLETELY different results if you don't fully parenthesise.
I cannot imagine when it would ever be important that those two pairs have differing precedence, but they do.  And I just spent two hours discovering that the hard way because I broke my career-long rule:
*Always fully parenthesise!*


webmaster

I regularly use some syntax like :
<?php
if(!$myvar)
 $myvar = $value ;
?>
and
<?php
if($myvar)
 echo "myvar is $myvar today" ;
?>
(or <?php echo ($myvar ? "myvar is $myvar today" : "") ?>)
It's small, but can become heavy when used too much.
Isn't there some trick to better such syntaxes ?
I was wondering about using things like :
<?php $myvar ||= $value ; ?>
<?php $myvar ?= $value ; ?>
<?php echo ($myvar ? "myvar is $myvar today") ; ?>


kit dot lester

D'oh! please ignore (& forgive) the first paragraph in my note yesterday - I said that a diadic operator had the same precedence & associativity as a bunch of monadics. A bit of early senility must have struck me.
When I find time I'll rework my test to find out what I should have said - but using instanceof monadically didn't cause any visible errors in the test, so it could take a while.
[I'd be delighted if someone else beats me to it/ spares me the difficulties of seeing what's wrong in something that shouldn't have worked.]


yasuo_ohgaki

About "{" and "}".
Sometimes PHP programmers need to use "{" and "}" to resolve ambiguity. Here is some examples.
Variable Variables:
$foo = "test";
$$bar = "this is";
echo "${$bar} $foo"; // prints "this is test"
Note: it is the same as
echo "$test $foo";
Array in string:
$arr[10][10][10] = "string";
echo "This is {$arr[10][10][10]}"; // prints "This is string"
Associative Array in string:
$arr['item'] = 'string';
echo "This is {$arr['item']}"; //prints "This is string".


rick

A quick note to any C developers out there, assignment expressions are not interpreted as you may expect - take the following code ;-
<?php
$a=array(1,2,3);
$b=array(4,5,6);
$c=1;
$a[$c++]=$b[$c++];
print_r( $a ) ;
?>
This will output;-
Array ( [0] => 1 [1] => 6 [2] => 3 )
as if the code said;-
$a[1]=$b[2];
Under a C compiler the result is;-
Array ( [0] => 1 [1] => 5 [2] => 3 )
as if the code said;-
$a[1]=$b[1];
It would appear that in php the increment in the left side of the assignment is processed prior to processing the right side of the assignment, whereas in C, neither increment occurs until after the assignment.


gautam

<?php
$result1 = 7 + 8 * 9/3 -4;
$result2 = 7 + 8 * (9/3 -4);
$result3 =(7 + 8)* 9/3 -4;
echo "Result1 for 7 + 8 * 9/3 -4 = $result1  Result2 for 7 + 8 * (9/3 -4) = $result2 and Result3 (7 + 8)* 9/3 -4 = $result3 "
/*
which gives results as under
Result1 for 7 + 8 * 9/3 -4 = 27 Result2 for 7 + 8 * (9/3 -4) = -1 and Result3 (7 + 8)* 9/3 -4 = 41
Execution Order is 1) expression in brackets 2) division 3) multiplication 4) addition and 5) subtraction
*/
?>


mathiasrav

<?php
"This might be related to the comment 31-Oct-2006 07:20 just below, but here goes.";
"I have found a bug (might be inherited from the way C does it) in PHP's operator precedence. Things like:";
if ($a != ($a = $b)) {/* more things */}
"doesn't work. In my code, I've changed that to:";
if ($a != $b) {$a = $b; /* more things */}
"A longer explanation (and a practical example) follows.";
"I've written an IRC bot which (among other things) can parse an IRC string and find out how the individual characters are formatted.";
// Quick guide to IRC formatting: In IRC, there are several 'format control characters', all non-printable. Among these are the bold formatting character, 0x02, which I'll write as <b>, and the underline formatting, 0x1F, which I'll write as <u>.";
"My code parses a string and reads these characters and makes an array which contains the individual characters and their formatting.";
"Here's an example. (I named the indices of the array, my code actually uses numbered indices)";
$ourstring = array(
 array(
   'Hello ',
   'bold' => 1,
   'underline' => 0,
 ), array(
   'world.',
   'bold' => 0,
   'underline' => 1,
 )
);
"And now, my formatparsedarray-to-ircstring function, that translates this to a string which can be posted to an IRC channel.";
$resstring = '';
define('bold', "\x02");
define('underline', "\x1F");
$bold = false;
$underline = false;
foreach ($ourstring as $c) {
 if ($bold != ($bold = $c['bold'])) $resstring .= bold;
 if ($underline != ($underline = $c['underline'])) $resstring .= underline;
 $resstring .= $c[0];
}
"Now, $resstring should contain:";
$resstring == "\x02Hello \x02\x1Fworld.";
"However, it doesn't, since ($bold = $c\['bold'\]) is evaluated before ($bold == (...)). According to the operator precedence, == is evaluated left-to-right, and = is evaluated right-to-left, but in this case, == evaluated right-to-left.";
"Is this a bug, or is it a leftover from C's way of parsing such things?";
?>


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