This seems trivial to do in userspace to me:

function which($cmd) {
	$paths = explode(':',$_ENV['PATH']);
	foreach($paths as $path) {
		$p = $path.'/'.$cmd;
		if(file_exists($p)) return $p;
	}
	return false;
}

Thanks for your quick reply. I agree - it's trivial to do in userspace. 

The simplest way is just to 
 $lastline = exec ("which $ffmpeg", $output, $retval)
 if (retval){
   return ($lastline)
 }else{
   return ($false)
 } 


Indeed many of the PHP functions are trivial in that sense - one could build file_get_contents() out of fopen(),file(),fclose(), or could avoid using unlink by a call to system("rm filename").

However the attraction of PHP is that so many of the required functions already exist and I don't have to write them. So I think that which() would be a useful addition. 

I'm particularly thinking of the cases where php-cli is an improvement on shell-scripting.

Using exec is definitely not the best way to do that one.  Anytime you fork+exec a 
new process you take on a significant performance penalty.  Same goes for your 
system() example.  But, if you can do it in a couple of lines without resorting to 
an exec(), then it is indeed trivial.  file_get_contents() is slightly more 
complicated because of the streams support in it.