<?php

function replace_between($str, $needle_start, $needle_end, $replacement) {
    $pos = strpos($str, $needle_start);
    $start = $pos === false ? 0 : $pos + strlen($needle_start);

    $pos = strpos($str, $needle_end, $start);
    $end = $start === false ? strlen($str) : $pos;
 
    return substr_replace($str,$replacement,  $start, $end - $start);
}

$string = '<div class="header-top clearfix">Текст</div>';

echo replace_between($string, '<div class="header-top clearfix">', '</div>', 'Замененный текст');

<div class="header-top clearfix">Замененный текст</div>