PHP implementation of the Karatsuba algorithm (base 10)

karatsuba.php

<?php

function karatsuba($num1, $num2) {

  if(($num1 < 10) || ($num2 < 10)) {
    return $num1 * $num2;
  }
  
  $m = ceil((max(ceil(log10($num1)), ceil(log10($num2))))/2);
  
  $sn1   = "".$num1;
  $high1 = (int)substr($sn1, 0, strlen($sn1) - $m);
  $low1  = (int)substr($sn1, -$m);
  
  $sn2   = "".$num2;
  $high2 = (int)substr($sn2, 0, strlen($sn2) - $m);
  $low2  = (int)substr($sn2, -$m);
  
  $z0 = karatsuba($low1, $low2);
  $z1 = karatsuba($low1 + $high1, $low2 + $high2);
  $z2 = karatsuba($high1, $high2);
  
  return ($z2 * pow(10, 2 * $m)) + (($z1 - $z2 - $z0) * pow(10, $m)) + $z0;
}

header("Content-type: text/plain");

$n1 = isset($_GET['n1']) ? $_GET['n1'] : mt_rand(0, mt_getrandmax());
$n2 = isset($_GET['n2']) ? $_GET['n2'] : mt_rand(0, mt_getrandmax());
$k  = karatsuba($n1, $n2);

printf("%d * %d = %d\n", $n1, $n2, $k);

?>