<?php
$rumus1 = "tan('a')*('Wo'+2)";

$k[0] = 'a';
$k[1] = 'Wo';

$v[0] = 2;
$v[1] = 3;

$res = str_replace($k, $v, $rumus1);
eval("\$final = $res;");
echo $final;
?>