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 NULLwhen passed incompatible parameters. There are
     some exceptions to this rule, such as the get_class()
     function, which will continue to returnFALSEon 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_FILEandPDO::MYSQL_ATTR_READ_DEFAULT_GROUPare 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.