- str_replace: documentation ( source)
- trim: documentation ( source)
<?php
$html = <<<HTML
<div class="checkbox">Dynamic_String_Exemple<span class="counter">1</span></div>
HTML;
$dom = new DOMDocument;
$dom->loadHTML($html, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
$firstDivFirstNode = $dom->getElementsByTagName('div')->item(0)->childNodes->item(0);
$firstDivFirstNode->nodeValue = str_replace(' ', '_', $firstDivFirstNode->nodeValue);
echo trim($dom->saveHTML());