DOMDocument and UTF8. MySQL says: Incorrect string value - php

Question

I am trying to load the meta description of this website (which has a German character) via the following script in PHP:
$page_content = file_get_contents($uri);
$dom_obj = new \DOMDocument();
$dom_obj->loadHTML(mb_convert_encoding($page_content, 'HTML-ENTITIES', 'UTF-8'));
However, while trying to write it into the MySQL db, Laravel says it ran into troubles trying to write that into the db: incorrect string value "\xC3" (which is the German character)
When I simply do the following, writing to the db works. But the character is not displayed correctly (ü instead of ü)
$dom_obj->loadHTML($page_content)
This problem only occurs with this website so far, others I tried with the same character do work. Can you think of a possible reason and fix? Thank you!
Edit:
It works fine, when I use PHPs "utf8_decode" to decode the meta description that I get via $dom_obj without mb_convert_encoding. When I do this, all other sites that worked before lead to errors (like this: Incorrect string value: '\xE4t')

Answer 1

I found the error. I was using substr to shorten the description. Apparently substr cut off one of those special characters and this is why it wasnt working.
foreach($dom_obj->getElementsByTagName('meta') as $meta) {
if($meta->getAttribute('name')=='description'){
substr($meta->getAttribute('content'), 0, 156);
This is a workaround:
mb_substr($foo,0,156,"UTF-8");