preg_replace('/(<a.*?)class=".*?"/', '$1', $tekse);

<?php
$dom = new DOMDocument();
$dom->loadHTML( '<a href="https://www.comandeer.pl" class="whatever">Test 1</a>
<a href="https://www.comandeer.pl" class=whatever>Test 2</a>
<a href="https://www.comandeer.pl" CLasS="whatEver">Test 3</a>
<div class="aha">Test 4</div>' );

$as = $dom->getElementsByTagName( 'a' );

foreach ( $as as $a ) {
    $a->removeAttribute( 'class' );
}

var_dump( $dom->saveHTML() );