SQL knowledge and idea about Indexing, Joins, Sub Queries,Relational databases in mysql, Storage Engines like: MYISAM & INNODB
PDO OR MYSQLI and storage procedure of data
Valuable command on Frameworks based on MVC pattern
Core knowledge of PHP or JS things like:
a. Version Update and Importance/Improvements in each
b. Inbuilt PHP array & string function
c. PHP Profilers
d. Clear Knowledge about exceptional handing
e. Debugging, Memory Allocation, Deallocation and many more.
Scale and Big Data problems and solutions available especially when it comes to searching.