What has changed in PHP 5.3.x
New features
Migrating from PHP 5.2.x to PHP 5.3.x
PHP Manual

## Backward Incompatible Changes

Although most existing PHP 5 code should work without changes, please take note of some backward incompatible changes:

- The newer internal parameter parsing API has been applied across all the extensions bundled with PHP 5.3.x. This parameter parsing API causes functions to return `NULL` when passed incompatible parameters. There are some exceptions to this rule, such as the get_class() function, which will continue to return `FALSE` on error.
- clearstatcache() no longer clears the realpath cache by default.
- realpath() is now fully platform-independent. Consequence of this is that invalid relative paths such as __FILE__ . "/../x" do not work anymore.
- The call_user_func() family of functions now propagate $this even if the callee is a parent class.
- The array functions natsort(), natcasesort(), usort(), uasort(), uksort(), array_flip(), and array_unique() no longer accept objects passed as arguments. To apply these functions to an object, cast the object to an array first.
- The behaviour of functions with by-reference parameters called by value has changed. Where previously the function would accept the by-value argument, a fatal error is now emitted. Any previous code passing constants or literals to functions expecting references, will need altering to assign the value to a variable before calling the function.
- The new mysqlnd library necessitates the use of MySQL 4.1's newer 41-byte password format. Continued use of the old 16-byte passwords will cause mysql_connect() and similar functions to emit the error, "mysqlnd cannot connect to MySQL 4.1+ using old authentication."
- The new mysqlnd library does not read mysql configuration files (my.cnf/my.ini), as the older libmysqlclient library does. If your code relies on settings in the configuration file, you can load it explicitly with the mysqli_options() function. Note that this means the PDO specific constants `PDO::MYSQL_ATTR_READ_DEFAULT_FILE` and `PDO::MYSQL_ATTR_READ_DEFAULT_GROUP` are not defined if MySQL support in PDO is compiled with mysqlnd.
- The trailing / has been removed from the SplFileInfo class and other related directory classes.
- The __toString() magic method can no longer accept arguments.
- The magic methods __get(), __set(), __isset(), __unset(), and __call() must always be public and can no longer be static. Method signatures are now enforced.
- The __call() magic method is now invoked on access to private and protected methods.
- func_get_arg(), func_get_args() and func_num_args() can no longer be called from the outermost scope of a file that has been included by calling include or require from within a function in the calling file.
- An emulation layer for the MHASH extension to wrap around the Hash extension have been added. However not all the algorithms are covered, notable the s2k hashing algorithm. This means that s2k hashing is no longer available as of PHP 5.3.0.

The following keywords are now reserved and may not be used in function, class, etc. names.

- goto
- namespace

What has changed in PHP 5.3.x
New features
Migrating from PHP 5.2.x to PHP 5.3.x
PHP Manual