Which app can I learn online and get a professional certificate?

A lot. The certification is a certification, you must create a portfolio of sites or programs to Github to prove that you know.There are a lot of online schools. You can be certified for many like:

• WordPress
• Joomla
• Drupal
• CodeIgniter
• Laravel
• PHP
• HTML
• CSS
• MYSQL
• JS
• Photoshop
• Gimp
• Magento
• Moodle
• and more

and more. There are technical sites and university sites. I have followed courses from the:

• OStraining (CSM tech)
• Joomlashack (Joomla)
• VTC (good summary courses)
• Eduonix
• Udemy (pay and have the course forever + updates)

You must decide your area, check the market, and study.