feat: php opening tags (closes #12)

[Zentreax]

Hey! 👋

In this PR i adressed issue #12.

I added PHP opening tags for every file but "embed_responsive_16_to_9_videos.php" because I don't know if the CSS should be commented out or not.

I also didn't add closing tags:
"If a file contains only PHP code, it is preferable to omit the PHP closing tag at the end of the file. This prevents accidental whitespace or new lines being added after the PHP closing tag, which may cause unwanted effects because PHP will start output buffering when there is no intention from the programmer to send any output at that point in the script. "
(source: https://www.php.net/manual/en/language.basic-syntax.phptags.php)

Would be great if you can accept this for hacktoberfest ;)

• Zentreax

[Zentreax]

And can you label my PR as 'hacktoberfest-accepted' please? Since this repository as a whole isn't participating in hacktoberfest.

[dalenguyen]

Thanks, @Zentreax!