PHP multidimensional array to one-dimensional array

Since many times, we need to turn multidimensional arrays into one-dimensional arrays, because we only need one-dimensional arrays, and one-dimensional arrays are more convenient to use, how do you convert multidimensional arrays into one-dimensional arrays in PHP?
<?php
function _rebuild_array($arr){  //rebuild a array
  static $tmp=array();

  for($i=0; $i<count($arr); $i++){
    if(is_array($arr[$i])) _rebuild_array($arr[$i]);
    else $tmp[]=$arr[$i];
  }

  return $tmp;
}
?>Here's an example to verify:
<?php
$arr=array(‘123.html‘,‘456.html‘,array(‘dw.html‘,‘fl.html‘,array(‘ps.html‘,‘fw.html‘)),‘ab.html‘);
//定义一个三维数组，用来检测我们的函数
echo ‘<pre>‘;
print_r(_rebuild_array($arr));
echo ‘</pre>‘;

PHP multidimensional array to one-dimensional array