Find a substring within a string and return the index where it was found

$bigString = "I am a really big realtor in the bay area";
$pattern = "realtor";

$i = 0;
$j = 0;
$bigStringLen = strlen($bigString);
$patternLen = strlen($pattern);
$found = false;
while ($i <= ($bigStringLen - $patternLen)) {
   if ($bigString[$i] == $pattern[$j]) {

   } else {
       $j=0;
   }
   $i++; $j++;
   if ($j >= $patternLen) {
       $found = true;
       break;
   }
}
if ($found) {
    print "found at " . ($i-$patternLen) . "\n";
} else {
    echo "\nnot found\n";
}