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)){
      echo "working on $fl\n";
      $content = file_get_contents($fl);
      $content = str_replace('mysqli_query($conn, $query)', 'mysqli_query($conn, $query)', $content);
      file_put_contents($fl, $content);
      echo "Corrected $fl\n\n";
    }
  }
}
echo 'done';
?>