<?php
$number = $_POST['amount'];
$length = strlen($number);
for ($i=0; $i<$length; $i++)
{
    if ( !is_numeric($number[$i]) ) $number[$i] = '';
}
?>

$number = $_POST['amount'];
$pattern = '/[^0-9]/'; // This means to search for anything that is not a number
$string = preg_replace($pattern, '', $number); // Replaces anything that matches the pattern with an empty string

// String has now been stripped of all non numeric characters

function strip_to_numbers_only($string)
{
     $pattern = '/[^0-9]/';
     return preg_replace($pattern, '', $string);
}

// Call the function like this
$string = '9034klfa9032jklfau2hhf234';
$string = strip_to_numbers_only($string);

<?php

function strip_non_numeric($string)
{
  $pattern = '/[^0-9]/';
  return preg_replace($pattern, '', $string);
}

function loop_non_numeric($string)
{
    $numbers = '';
    for($i = 0; $i < strlen($string); $i++){
        $temp = ord(substr($string, $i, 1));
        if($temp > 47 && $temp < 58){
            $numbers .= chr($temp);
        }
    }
    return $numbers;
}

function microtime_float()
{
  list($usec, $sec) = explode(" ", microtime());
  return ((float)$usec + (float)$sec);
}

function benchmark($string)
{
  echo "String length: " . strlen($string) . "<br />";
  $start = microtime_float();
  for ($i=0; $i<100000; $i++)
  {
    $temp = strip_non_numeric($string);
  }
  $end = microtime_float();
  echo "Strip took: " . ($end-$start) . " Seconds<br />";

  $start = microtime_float();
  for ($i=0; $i<100000; $i++)
  {
    $temp = loop_non_numeric($string);
  }
  $end = microtime_float();
  echo "Loop took: " . ($end-$start) . " Seconds<br /><br />";
}

$string = '123k';
benchmark($string);

$string = '348jfakl324928249majsf;l24;lkja8902340$%df@#';
benchmark($string);

$string = '348jfakl324928249majsf;l24;lkja8902340$%df@#348jfakl324928249majsf;l24;lkja8902340$%df@#';
benchmark($string);

?>