Applying a CSS whitelist to HTML in PHP [closed]

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Lets say I have the following $string...

<span style='text-decoration:underline; display:none;'>Some text</span>

I only want to allow the style text-decoration, so I want a PHP function like the following...

$string = stripStyles($string, array("text-decoration"));

Similar to strip_tags, but using an array instead. So $string will now be...

<span style='text-decoration:underline;'>Some text</span>

I am using Cake, so if this can be done with Sanitize then all the better.

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This is tricky, but you should be able to do it with DOMDocument. This should get you started, but it's likely to require some serious tweaking.

// Load your html string
$dom = new DOMDocument();
$dom->loadHTML($your_html_string);

// Get all the <span> tags
$spans = $dom->getElementsByTagName("span");

// Loop over the span tags
foreach($spans as $span) {

  // If they have a style attribute that contains "text-decoration:"
  // attempt to replace the contents of the style attribute with only the text-decoration component.
  if ($style = $span->getAttribute("style")) {
    if (preg_match('/text-decoration:([^;]*);/i', $style)) {
      $span->setAttribute("style", preg_replace('/^(.*)text-decoration:([^;]*);(.*)$/i', "text-decoration:$2;", $style);
    }
    // Otherwise, erase the style attribute
    else $span->setAttribute("style", "");
  }
}

$output = $dom->saveHTML;

It's maybe better to attempt to parse the style attributes by explode()ing on ;

// This replaces the inner contents of the foreach ($spans as $span) above...

// Instead of the preg_replace()
$styles = explode(";", $style);
$replaced_style = FALSE;
foreach ($styles as $s) {
 if (preg_match('/text-decoration/', $s) {
   $span->setAttribute("style", $s);
   $replaced_style = TRUE;
 }
 //  If a text-decoration wasn't found, empty out the style
 if (!$replaced_style) $span->setAttribute("style", "");
}