Replace mysql to mysqli within a directory hierarchy

script.php

<?php
$folders = [
  '*.php',
  '*/*.php',
  '*/*/*.php'
];
foreach ($folders as $f){
  $res = shell_exec("ls $f");
  $files = explode("\n", $res);
  echo("\n");
  foreach ($files as $fl){
    if(is_file($fl) && pathinfo($fl, PATHINFO_EXTENSION) === "php"){
      echo "working on $fl\n";
      $content = file_get_contents($fl);
      $old = ["mysql_query(", "mysql_fetch", "mysql_num", "mysql_error()"];
      $new = ['mysqli_query($con, ', "mysqli_fetch", "mysqli_num", 'mysqli_error($con)'];
      $content =  str_replace($old, $new, $content);
      file_put_contents($fl, $content);
      echo "Corrected $fl\n\n";
    }
  }
}
echo 'done';
?>