[kh99]

I'm not completely sure what you're askking, but you could do this:

$display = 'hello word - hello word - hello word';
$find = 'h'; 
$replace = array('1','2','3','4');
$output = str_replace(array_fill(0, count($replace), $find), $replace, $display);

or if you want random replacements:

$output = str_replace(array_fill(0, count($replace), shuffle($find)), $replace, $display);

That will use them in a random order. If instead you want each replacement to have the same chance of being used each time (in other words, you could get h1ello word - h2ello word - h1ello word) then I think you need something a little more complicated.