Backward Incompatible Changes

Although most existing PHP 4 code should work without changes, you should pay attention to this backward imcompatible changes:

strrpos() and strripos() now use the entire string as a needle.
Ilegal use of string offsets causes E_ERROR instead of E_WARNING.
array_merge() was changed to accept only arrays. If a non-array variable is passed, a E_WARNING will be thrown for every such parameter. Be careful because your code may start emitting E_WARNING out of the blue.
PATH_TRANSLATED server variable is no longer set implicitly under Apache2 SAPI in contrast to the situation in PHP 4, where it is set to the same value as the SCRIPT_FILENAME server variable when it is not populated by Apache. This change was made to comply with the CGI specification. Please refer to bug #23610 for further information.
The T_ML_CONSTANT constant is no longer defined by the Tokenizer extension. If error_reporting is set to E_ALL, PHP will generate a notice. Although the T_ML_CONSTANT was never used at all, it was defined in PHP 4. In both PHP 4 and PHP 5 // and /* */ are resolved as the T_COMMENT constant. However the PHPDoc style comments /** */ ,which starting PHP 5 are parsed by PHP, are recognized as T_DOC_COMMENT.
$_SERVER should be populated with argc and argv if variables_order includes "S". If you have specifically configured your system to not create $_SERVER, then of course it shouldn't be there. The change was to always make argc and argv available in the CLI version regardless of the variables_order setting. As in, the CLI version will now always populate the global $argc and $argv variables.
An object with no properties is no longer considered "empty".
Classes must be declared before used.

Example B-1. strrpos() and strripos() now use the entire string as a needle

 <?php
var_dump(strrpos('ABCDEF','DEF')); //int(3)

var_dump(strrpos('ABCDEF','DAF')); //bool(false)
?>

Example B-2. An object with no properties is no longer considered "empty"

 <?php
class test { }
$t = new test();

var_dump(empty($t)); // echo bool(false)

if (!$t) {
    // Will be executed
}
?>

The following example was valid in PHP 4, although it will produce a fatal error in PHP 5.

Example B-3. Classes must be declared before used

 <?php
$test = new fubar();
$test->barfu();

class fubar {
    function barfu() {
        echo 'fubar';
    }
}
?>