php - Get all value of h1 tag

I want to get all value of h1 tag and I found this article: getting all values from h1 tags using php

Then I tried to using:

include "functions/simple_html_dom.php";

function getTextBetweenTags($string, $tagname) {
    // Create DOM from string
    $html = str_get_html($string);

    $titles = array();
    // Find all tags 
    foreach($html->find($tagname) as $element) {
        $titles[] = $element->plaintext;
    }
    return $titles;

}
echo getTextBetweenTags("http://mydomain.com/","h1");
?>

But it's not running and I get:

Notice: Array to string conversion in C:\xampp\htdocs\checker\abc.php
on line 14 Array

Plz help me fix it. I want to get all h1 tag of a website with input data is URL of that website. Thanks so much!