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PHP : Appendices : PHP type comparison tables

Appendix R. PHP type comparison tables

The following tables demonstrate behaviors of PHP types and comparison operators, for both loose and strict comparisons. This supplemental is also related to the manual section on type juggling. Inspiration was provided by various user comments and by the work over at » BlueShoes.

Before utilizing these tables, it's important to understand types and their meanings. For example, "42" is a string while 42 is an integer. FALSE is a boolean while "false" is a string.

Note:

HTML Forms do not pass integers, floats, or booleans; they pass strings. To find out if a string is numeric, you may use is_numeric().

Note:

Simply doing if ($x) while $x is undefined will generate an error of level E_NOTICE. Instead, consider using empty() or isset() and/or initialize your variables.

Table R.1. Comparisons of $x with PHP functions

Expression gettype() empty() is_null() isset() boolean : if($x)
$x = ""; string TRUE FALSE TRUE FALSE
$x = null; NULL TRUE TRUE FALSE FALSE
var $x; NULL TRUE TRUE FALSE FALSE
$x is undefined NULL TRUE TRUE FALSE FALSE
$x = array(); array TRUE FALSE TRUE FALSE
$x = false; boolean TRUE FALSE TRUE FALSE
$x = true; boolean FALSE FALSE TRUE TRUE
$x = 1; integer FALSE FALSE TRUE TRUE
$x = 42; integer FALSE FALSE TRUE TRUE
$x = 0; integer TRUE FALSE TRUE FALSE
$x = -1; integer FALSE FALSE TRUE TRUE
$x = "1"; string FALSE FALSE TRUE TRUE
$x = "0"; string TRUE FALSE TRUE FALSE
$x = "-1"; string FALSE FALSE TRUE TRUE
$x = "php"; string FALSE FALSE TRUE TRUE
$x = "true"; string FALSE FALSE TRUE TRUE
$x = "false"; string FALSE FALSE TRUE TRUE


Table R.2. Loose comparisons with ==

TRUE FALSE 1 0 -1 "1" "0" "-1" NULL array() "php" ""
TRUE TRUE FALSE TRUE FALSE TRUE TRUE FALSE TRUE FALSE FALSE TRUE FALSE
FALSE FALSE TRUE FALSE TRUE FALSE FALSE TRUE FALSE TRUE TRUE FALSE TRUE
1 TRUE FALSE TRUE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE
0 FALSE TRUE FALSE TRUE FALSE FALSE TRUE FALSE TRUE FALSE TRUE TRUE
-1 TRUE FALSE FALSE FALSE TRUE FALSE FALSE TRUE FALSE FALSE FALSE FALSE
"1" TRUE FALSE TRUE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE
"0" FALSE TRUE FALSE TRUE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE
"-1" TRUE FALSE FALSE FALSE TRUE FALSE FALSE TRUE FALSE FALSE FALSE FALSE
NULL FALSE TRUE FALSE TRUE FALSE FALSE FALSE FALSE TRUE TRUE FALSE TRUE
array() FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE TRUE TRUE FALSE FALSE
"php" TRUE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE TRUE FALSE
"" FALSE TRUE FALSE TRUE FALSE FALSE FALSE FALSE TRUE FALSE FALSE TRUE


Table R.3. Strict comparisons with ===

TRUE FALSE 1 0 -1 "1" "0" "-1" NULL array() "php" ""
TRUE TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
1 FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
0 FALSE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
-1 FALSE FALSE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
"1" FALSE FALSE FALSE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE FALSE
"0" FALSE FALSE FALSE FALSE FALSE FALSE TRUE FALSE FALSE FALSE FALSE FALSE
"-1" FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE FALSE FALSE FALSE FALSE
NULL FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE FALSE FALSE FALSE
array() FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE FALSE FALSE
"php" FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE FALSE
"" FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE TRUE


PHP 3.0 note:

The string value "0" was considered non-empty in PHP 3, this behavior changed in PHP 4 where it's now seen as empty.

Code Examples / Notes » types.comparisons

aidan
The way PHP handles comparisons when multiple types are concerned is quite confusing.
For example:
"php" == 0
This is true, because the string is casted interally to an integer. Any string (that does not start with a number), when casted to an integer, will be 0.

omit
the manual said "HTML Forms do not pass integers, floats, or booleans; they pass strings"
while this is true, php will sometimes change the type to either type array, or possibly type integer(no, not a numeric string) if it was used as an array key. php seems to do this when it parses the request data into the predefined variable arrays.
example:
<input type="text" name="foo[5]">
<input type="text" name="foo[7]">
now obviously the browser will send those names as a string. but php will change thier type.
<?php
// $_POST['foo'] is an array
var_dump($_POST['foo']);
foreach ($_POST['foo'] as $key => $val) {
   // the keys 5 and 7 will be type integer
   var_dump($key);
}
?>
because of this, its also a good idea to check the types of your variables.

15-mar-2007 10:06
Re: omit's comment
The note omit quotes is referring to the VALUE returned, not its name. If you put 42 into a text field, the corresponding array value will be the string "42". The note makes no comment on the array's keys.

jan
Note that php comparison is not transitive:
"php" == 0 => true
0 == null => true
null == "php" => false

jerryschwartz
In some languages, a boolean is promoted to an integer (with a value of 1 or -1, typically) if used in an expression with an integer. I found that PHP has it both ways:
If you add a boolean with a value of true to an integer with a value of 3, the result will be 4 (because the boolean is cast as an integer).
On the other hand, if you test a boolean with a value of true for equality with an integer with a value of three, the result will be true (because the integer is cast as a boolean).
Surprisingly, at first glance, if you use either < or > as the comparison operator the result is always false (again, because the integer as cast as a boolean, and true is neither greater nor less than true).

php
In case it helps someone, here's a table to compare different Variable tests (created before I found this page!)
http://www.deformedweb.co.uk/php_variable_tests.php

frank
A comparison table for <=,<,=>,> would be nice...
Following are TRUE (tested PHP4&5):
NULL <= -1
NULL <= 0
NULL <= 1
!(NULL >= -1)
NULL >= 0
!(NULL >= 1)
That was a surprise for me (and it is not like SQL, I would like to have the option to have SQL semantics with NULL...).

tom
<?php
if (strlen($_POST['var']) > 0) {
   // form value is ok
}
?>
When working with HTML forms this a good way to:
(A) let "0" post values through like select or radio values that correspond to array keys or checkbox booleans that would return FALSE with empty(), and;
(B) screen out $x = "" values, that would return TRUE with isset()!
Because HTML forms post values as strings, this is a good way to test variables!
[[Editor Note: This will create a PHP Error of level E_NOTICE if the checked variable (in this case $_POST['var']) is undefined. It may be used after (in conjuection with) isset() to prevent this.]]

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