- var_dump: documentation ( source)
- preg_match: documentation ( source)
<?php
$html=<<<'HTML'
<div class="wrapper">
<div class="some_useless_div">
<p>Some useless text paragraph.</p>
<div id="another_useless_div">
<p>The actual important part is: SOME_DYNAMIC_TEXT what I want to put into a variable. The text around that dynamic text is static text and will not change.</p>
</div>
</div>
</div>
HTML;
preg_match('/The actual important part is\: (.*?) what I want to put into a variable\./',$html,$matches);
$str=$matches[1];
var_dump($str);