(PECL runkit >= 0.7.0)
runkit_function_redefine — Replace a function definition with a new implementation
$funcname
   , string $arglist
   , string $code
   )Note: By default, only userspace functions may be removed, renamed, or modified. In order to override internal functions, you must enable the runkit.internal_override setting in php.ini.
funcname
     Name of function to redefine
arglist
     New list of arguments to be accepted by function
code
     New code implementation
   Returns TRUE on success or FALSE on failure.
  
Example #1 A runkit_function_redefine() example
<?php
function testme() {
  echo "Original Testme Implementation\n";
}
testme();
runkit_function_redefine('testme','','echo "New Testme Implementation\n";');
testme();
?>
The above example will output:
Original Testme Implementation New Testme Implementation