Get number of parameters a function requires

Only way is with reflection by going to http://us3.php.net/manual/en/book.reflection.php

class foo {
 function bar ($arg1, $arg2) {
   // ...
 }
}

$method = new ReflectionMethod('foo', 'bar');
$num = $method->getNumberOfParameters();

Use Reflection, especially ReflectionFunction in your case.

$fct = new ReflectionFunction('client_func');
echo $fct->getNumberOfRequiredParameters();

As far as I can see you will find getParameters() useful too