PHP parser to scrape data error - php

Question

I'm trying to code a php parser to gather professor reviews from ratemyprofessor.com. Each professor has a page and it has all the reviews in it, I want to parse each professor's site and extract the comments into a txt file.
This is what I have so far but it doesn't excute properly when I run it because the output txt file remains empty. what can be the issue?
<?php
set_time_limit(0);
$domain = "http://www.ratemyprofessors.com";
$content = "div id=commentsection";
$content_tag = "comment";
$output_file = "reviews.txt";
$max_urls_to_check = 400;
$rounds = 0;
$reviews_stack = array();
$max_size_domain_stack = 10000;
$checked_domains = array();
while ($domain != "" && $rounds < $max_urls_to_check) {
$doc = new DOMDocument();
#$doc->loadHTMLFile($domain);
$found = false;
foreach($doc->getElementsByTagName($content_tag) as $tag) {
if (strpos($tag->nodeValue, $content)) {
$found = true;
break;
}
}
$checked_domains[$domain] = $found;
foreach($doc->getElementsByTagName('a') as $link) {
$href = $link->getAttribute('href');
if (strpos($href, 'http://') !== false && strpos($href, $domain) === false) {
$href_array = explode("/", $href);
if (count($domain_stack) < $max_size_domain_stack &&
$checked_domains["http://".$href_array[2]] === null) {
array_push($domain_stack, "http://".$href_array[2]);
}
};
}
$domain_stack = array_unique($domain_stack);
$domain = $domain_stack[0];
unset($domain_stack[0]);
$domain_stack = array_values($domain_stack);
$rounds++;
}
$found_domains = "";
foreach ($checked_domains as $key => $value) {
if ($value) {
$found_domains .= $key."\n";
}
}
file_put_contents($output_file, $found_domains);
?>

Answer 1

This is what I have so far but it doesn't excute properly when I run it because the output txt file remains empty. what can be the issue?
It gives empty output since there is a lack of array variable initialization.
Main part. Add an initialization of variable:
$domain_stack = array(); // before while ($domain != ...... )
Additional. Fix other warnings and notices:
// change this
$checked_domains["http://".$href_array[2]] === null
// into
!isset($checked_domains["http://".$href_array[2]])
// another line
// check if key exists
if (isset($domain_stack[0])) {
$domain = $domain_stack[0];
unset($domain_stack[0]);
}