how search for a substring found within a string of text and highlight all of it

<div><span style="background: #f00">My hair cut so cool!</span></div>

0:
    1:  $term = 'cool';
    2:  ob_start();
    3:

   10:  foreach($items as $item) {
   11:    // echoing results here
   12:    echo '<div>' . $item->text . '</div>';
   13:  }

   30:  $content = ob_get_contents();
   31:  ob_clean() ;
   32:
   33:  $pattern = "/(?<!<[^>])($term)/i";
   34:  $replace = "<span style="background: #f00">$1</span>";
   35:  echo preg_replace($pattern, $replace, $content);
   36:

$pattern = "/[^<>]*$term[^<>]*/i";
$replace = "<span style="background: #f00">$0</span>";