Post installation message: Wrong PHP detection
From Joomla! Documentation
After an update to Joomla 3.7.3 the following postinstall message is displayed to all users.
Your PHP Version Will Be Unsupported in Joomla! 3.3 Since version 3.2.2 Beginning with Joomla! 3.3, the version of PHP this site is using will no longer be supported. Joomla 3.3 will require at least PHP version 5.3.10 in order to provide enhanced security features to its users.
This pertains only to Joomla! version(s): 3.7.3
What is the cause
Issue has been reported: https://github.com/joomla/joomla-cms/issues/16964
How to fix
See https://github.com/joomla/joomla-cms/pull/16974 or wait for Joomla! 3.7.4