PHP

• php.net | PHP Official website
• php.net/supported-versions | PHP Supported Versions
• packagist.org | The PHP Package Repository
• PHP-FIG | PHP standards recommendations
• 3v4l.org | Online PHP editor

REGEX

• regex101.com | Online regex tester, debugger with highlighting for PHP, PCRE, Python, Golang and JavaScript.

JSON

• JSON:API | A specification for building APIs in JSON

Markdown

• John Gruber's original spec | John Gruber's original spec
• php-markdown/extra | Markdown extra

CSS

• caniuse.com | Can I use provides up-to-date browser support tables for support of front-end web technologies on desktop and mobile web browsers

SSL

• certbot.eff.org | Automatically enable HTTPS on your website with EFF's Certbot, deploying Let's Encrypt certificates
• ssl-config.mozilla.org | Mozilla SSL Configuration Generator

Misc

• downforeveryoneorjustme.com | Check if a website is down for everyone or just you
• DNS Checker | Check DNS Propagation worldwide