1. strrpos() and strripos() now use the entire string as a needle. Be aware that the existing scripts may no longer work as you expect. EX : <?php var_dump(strrpos("ABCDEF","DEF")); var_dump(strrpos("ABCDEF","DAF")); ?> Will give you different results. The former returns 3 while the latter returns false rather than the position of the last occurrence of 'D'. The same applies to strripos(). 2. Illegal use of string offsets causes E_ERROR instead of E_WARNING. EX : <?php $a = "foo"; unset($a[0][1][2]); ?> Fatal error: Cannot use string offset as an array in ... on line 1 3. 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. 4. Be careful when porting from ext/mysql to ext/mysqli. The following functions return NULL when no more data is available in the result set (ext/mysql's functions return FALSE). - mysqli_fetch_row() - mysqli_fetch_array() - mysqli_fetch_assoc() 5. 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. 6. Starting PHP 5.0.0 the T_ML_CONSTANT constant is no longer defined by the ext/tokenizer extension. If error_reporting is set to E_ALL notices will be produced. Instead of T_ML_CONSTANT for /* */ the T_COMMENT constant is used, thus both // and /* */ are resolved as the T_COMMENT constant. However the PHPDoc style comments /** */ ,which starting PHP 5 are parsed by PHP, are recongnized as T_DOC_COMMENT. 7. $_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. 8. In some cases classes must be declared before used. It only happens only if some of the new features of PHP 5 are used. Otherwise the behaviour is the old. Example 1 (works with no errors): <?php $a = new a(); class a { } ?> Example 2 (throws an error): <?php $a = new a(); interface b{ } class a implements b { } ?> Output (example 2) : Fatal error: Class 'a' not found in /tmp/cl.php on line 2 9. get_class() starting PHP 5 returns the name of the class as it was declared which may lead to problems in older scripts that rely on the previous behaviour - the class name is lowercased. Expect the same behaviour from get_parent_class() when applicable. Example : <?php class FooBar { } class ExtFooBar extends FooBar{} $a = new FooBar(); var_dump(get_class($a), get_parent_class($a)); ?> Output (PHP 4): string(6) "foobar" string(9) "extfoobar" Output (PHP 5): string(6) "FooBar" string(9) "ExtFooBar" ---------------------------------------------------------------------- Example code that will break : //.... function someMethod($p) { if (get_class($p) != 'helpingclass') { return FALSE; } //... } //... Possible solution is to search for get_class() and get_parent_class() in all your scripts and use strtolower(). 10. get_class_methods() returns the names of the methods of a class as they declared. In PHP4 the names are all lowercased. Example code : <?php class Foo{ function doFoo(){} function hasFoo(){} } var_dump(get_class_methods("Foo")); ?> Output (PHP4): array(2) { [0]=> string(5) "dofoo" [1]=> string(6) "hasfoo" } Output (PHP5): array(2) { [0]=> string(5) "doFoo" [1]=> string(6) "hasFoo" } 11. Assignment $this is impossible. Starting PHP 5.0.0 $this has special meaning in class methods and is recognized by the PHP parser. The latter will generate a parse error when assignment to $this is found Example code : <?php class Foo { function assignNew($obj) { $this = $obj; } } $a = new Foo(); $b = new Foo(); $a->assignNew($b); echo "I was executed\n"; ?> Output (PHP 4): I was executed Output (PHP 5): PHP Fatal error: Cannot re-assign $this in /tmp/this_ex.php on line 4