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I'm trying to replace a word on every page for example "Denver" with a word from a URL string, so I'll have like ?city=Atlanta in the URL. So I was thinking of using PHP's GET to just get the city from the URL string and str_replace to replace it, but which action/hook in WordPress do I have to attach it to?

The word will appear throughout the entire page, including title, logo description, content, footer, etc... so it can't just replace content only.

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Check the Filter Reference -- there are filters like the_content, the_title, wp_title, etc. I'm not sure what you'd filter to get the logo description and footer -- you might need to delve into your theme's code.

Also, make sure you sanitize anything you get from $_GET -- never ever trust user-generated content. See Data Validation for more information.

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Dang I was hoping there would be a way to search all text and not have to go through individual elements. – zen May 3 '13 at 15:28
Each chunk of text -- the title, the content, etc -- is stored in a different field in the DB. But most everything in WordPress has a filter that you can use. Not all are documented in the Codex. Another site that lists filters and hooks -- though it doesn't explain them, by and large -- is Adam Brown's WordPress hooks database. – Pat J May 3 '13 at 15:54

str_replace will make a mess of any unlucky markup on the page since it will replace matching text inside of markup or inside of URLs.

What you want is a modified versions of an answer I gave to another question about highlighting search terms. The change would be to the highlight_search_term function. You just need to alter it to use the $_GET data instead of the search data.

function highlight_search_term($text){
    $keys = strip_tags($_GET['city']); // nominal validation
    $pattern = '/<[^>].*?>/i';
    $placeholders = array();
    foreach ($matches[0] as $v) {
      $placeholders[] = highlight_search_term_placeholders();
    $text = preg_replace_callback($pattern,'highlight_search_term_cb',$text);
    $pattern2 = '/(' . $keys .')/iu';
    $text = preg_replace($pattern2, ' <span class="search-term">\1</span> ', $text);
    $text = preg_replace($placeholders,$matches[0],$text);
  return $text;
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Does this still need to be attached to each filter, the_content, the_title, etc...? Also, where would I put which word I want to replace? If the target word is "Denver", title tag might contain "The Denver Website", so I'd only want to replace "Denver". – zen May 3 '13 at 15:26
Yes, it needs to be attached to the filters you want, at least. I only posted the altered code. You still need everything from the other answer. – s_ha_dum May 3 '13 at 15:31
Gotcha. I'll play around with it a bit and will update the post. – zen May 3 '13 at 15:45

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