First, PHP is a programming language and Mysql is a SGDB (database). Both are distinct and independent from each other.
The fact that you find many materials addressing both of them is because they are the most popular. Simply this.
Packets
What you will find most is materials with the LAMP or WAMP set:
L->Linux
A->Apache
M->MySQL
P->PHP
W->Windows
A->Apache
M->MySQL
P->PHP
Another popular set is XAMP
X->Windows, Linux, OS-X (mac)
A->Apache
M->MySQL
P->PHP
Why the LAMP set is so popular?
Simply because there are no licensing costs, they are open-source and can be used commercially.
But then you ask why there is little or almost no material for PGSQL (postgre)? After all it is also an open source database.
At this point we are talking about capitalism.
Capitalism, marketing
In an analogy, you have a bar where you offer 2 beers. One is popular and widely accepted the other is as good as but little known. To attract customers to your bar you will not put a poster on the door with the unknown brand of beer. Customers would go straight through and go to another bar where there is a poster with popular beer usually accompanied by the image of a famous person at the time.
That is, a book with title "php and mysql" will sell much more than a book "php postgre".
Hosting servers also give preference to the most popular and low-cost sets, in this case LAMP.
Mariadb
Obs: Currently you will find many packages encouraging Mariadb and discouraging the use of Mysql. This is a subject of long discussion and controversy so it is impossible to comment here. But basically Mariadb is Mysql. Born after a disagreement between the creator of Mysql and Oracle.
PHP is the programming language, mysql is the database. They usually go together because most applications need to persist data, ie, store them in an organized way. You can do this by other means, but the database is definitely the best choice. But it is not impossible to make an application with only the language.
– user28595
Because most people like to teach cake recipe instead of actually teach programming. More serious courses usually have more focus and teaches the foundation, teaches the person to think on their own, be creative, know how to solve any kind of problem, understand the basics to apply the right tool at the right time. But most prefer to be popular and this means giving people things ready to copy. It pleases more and "form" bad "professionals". Cake recipes need to be complete. Especially in PHP the fashion is to give the recipes and teach nothing.
– Maniero
For the ease of finding servers on the Internet that has support for these technologies, so there is a lot of material focusing on them 2. where a programming language , a database are important parts to make a form for example.
– Glaulher Medeiros