How Do I Take Out Only Numbers From A PHP String?

Correct variant will be:

$string = "Hello! 123 How Are You? 456";
$int = intval(preg_replace('/[^0-9]+/', '', $string), 10);

You can use this method to select only digit present in your text

function returnDecimal($text) {
    $tmp = "";  
    for($text as $key => $val) {
      if($val >= 0 && $val <= 9){
         $tmp .= $val
      }
    }
    return $tmp;
}