PHP and MySQL are two languages that complete each othe. Simply, with php you can create your application; and by including the MySQL your application will be more effecient.
For example, imagine that you want to create an application that allows you to register data such as “contacts information” where you will need to build a form that includes more than a field. Let say we need two fields one for contact’s name, the second one for contact’s number. For sure after users put the information they will have to submit the form. Submitting the form is dealing with PHP. However, after submitting the form what do you want the application to do with the information provided in the form. PHP can only show you the information. Of course you do not want the information shows up for a while and then disappears. In this situation we invite the MySQL to save the information for us as reference. The two languages complete each other in a way that PHP can send the information to MySQL database. Moreover, MySQL would also allows PHP to connect to the database and import the saved data.
The example above clearly explains the relationship between PHP and MySQL.