PHP 4’s call_user_func passes everything by value

I spent quite a while today debugging a problem where call_user_func was not passing a parameter by reference. I was trying to pass an object into a function whose name is not known until run time.

Passing it by reference means that changes made to $var inside foo() are made to the actual variable instead of to a copy of the value (when passed by value).  However, for some reason, when calling a function with call_user_func(), it passes everything by value, regardless of how the function is defined.

function foo(&$var)
{
  $var++;
}

$bar = 1;
foo($bar);
echo $bar;    // outputs '2'

$function = 'foo';

call_user_func($function, $bar);
echo $bar;  // you'd expect this to output 3 now, but it still outputs 2

$function($bar);
echo $bar;  // outputs 3 now

As the sample code shows, the solution is to avoid the use of the call_user_func() function by using a variable function name. Thanks to Steve Hannah’s blog post at http://www.sjhannah.com/blog/?p=86 for helping me to solve this one.