Modify Html Tag Attributes in the HTML String

modifyHtmlTagAttrsInString.php

<?php

/**
 * Modifies html tag attributes in the html string
 *
 * Remember, this will autofix the passed html. So if invalid html string is sent (e.g. `a` tag w/o end),
 * then the o/p returned by function will be valid html string.
 * 
 * Examples:
 * 1. modifyHtmlTagAttrsInString(
 *      '<a href="http://example.com" class="bg-gray-200" data-extra="present">Example Site</a>', 
 *      'a', 
 *      ['rel' => 'nofollow external', 'class' => '{EXISTING_VALUE} color-red']
 *  ); 
 * Above will return `<a href="http://example.com" class="bg-gray-200 color-red" data-extra="present" rel="nofollow external">Example Site</a>`
 * 
 * 2. modifyHtmlTagAttrsInString(
 *      '<a href="#""><span>Example Site</span></a>', 
 *      '//a/span', 
 *      ['class' => 'external-icon']
 *  ); 
 * Above will return `<a href="#"><span class="external-icon">Example Site</span></a>`
 * 
 * 3. modifyHtmlTagAttrsInString(
 *      '<a href="#">', 
 *      'a', 
 *      ['class' => 'link']
 *  ); 
 * Above will return `<a href="#" class="link"></a>`.
 * 
 * @param string $htmlString    html string in which we want to update tag attribute
 * @param string $htmlTagXPath  Query to search the html tag we are looking for. @see https://www.php.net/manual/en/domxpath.query.php
 * @param array $attrs          Attributes to be updated
 * @return void
 */
function modifyHtmlTagAttrsInString($htmlString, $htmlTagXPath, $attrs = []) {
    $dom = new DOMDocument;
    $dom->loadHTML(mb_convert_encoding($htmlString, 'HTML-ENTITIES', 'UTF-8'), LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);

    $xpath = new DOMXPath($dom);
    $htmlTagXPath = strpos($htmlTagXPath, '//') !== 0 ? "//{$htmlTagXPath}" : $htmlTagXPath;
    $nodes = $xpath->query($htmlTagXPath);

    foreach($nodes as $node) {
        foreach($attrs as $attrName => $attrValue) {
            $node->setAttribute($attrName, str_replace('{EXISTING_VALUE}', $node->getAttribute($attrName), $attrValue));
        }
    }

    return $dom->saveHTML();
}