<?
$string = "FOO STRING";
$string1 = "FOO";
$string2 = "OO";
$string3 = "ZOO";

$value1 = strpos($string,$string1,0);
$value2 = strpos($string,$string2,0);
$value3 = strpos($string,$string3,0);

for ($i=1;$i<=3;$i++) {
   if (${value.$i}>=0) {
      echo "string".$i." found at:".${value.$i}."<br>"; 
      }
   elseif (${value.$i}==FALSE) { 
      echo "string".$i." not found.<br>"; 
      }
}

?>

$value3>=0    and  $value3==FALSE   are equivalent  and so we can not distinguish between string found at position 0 and string not found.

strpos should return -1 when the string was not found.