Other incompatibilities

The PHP 3.0 Apache module no longer supports Apache versions prior to 1.2. Apache 1.2 or later is required.
echo() no longer supports a format string. Use the printf() function instead.
In PHP/FI 2.0, an implementation side-effect caused $foo[0] to have the same effect as $foo. This is not true for PHP 3.0.
Reading arrays with $array[] is no longer supported

That is, you cannot traverse an array by having a loop that does $data = $array[]. Use current() and next() instead.

Also, $array1[] = $array2 does not append the values of $array2 to $array1, but appends $array2 as the last entry of $array1. See also multidimensional array support.
"+" is no longer overloaded as a concatenation operator for strings, instead it converts it's arguments to numbers and performs numeric addition. Use "." instead.

echo "1" + "1"; ?>

In PHP 2.0 this would echo 11, in PHP 3.0 it would echo 2. Instead use:

echo "1"."1"; ?>

$a = 1;
$b = 1;

echo $a + $b; ?>

This would echo 2 in both PHP 2.0 and 3.0.

$a = 1;
$b = 1;

echo $a.$b; ?>

This will echo 11 in PHP 3.0.