Here, I'm listing some functions that do string manipulation in WordPress.

string_manipulations_wp.php

<?php
/**
 * Get a specific html tag from content
 * @return a specific HTML tag from the loaded content
 */
function rogan_get_html_tag( $tag = 'blockquote', $content = '' ) {
    $dom = new DOMDocument();
    $dom->loadHTML($content);
    $divs = $dom->getElementsByTagName( $tag );
    $i = 0;
    foreach ($divs as $div) {
        if($i == 1) {
            break;
        }
        echo "<{$div->tagName}> {$div->nodeValue} </{$div->tagName}>";
        ++$i;
    }
}