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