Since the release of WordPress 5.2, a new tool called "Site Health" is built into WordPress.
Using Site Health, you can check the version of PHP your WordPress site is running.
First, login to your WordPress Admin Dashboard.
Next, expand Tools and click Site Health.
Select the Info tab or go directly to this page by adding the following string to the end of your root URL: /wp-admin/site-health.php?tab=debug
Scroll down and expand the Server section where you’ll find your PHP version: