Fatal error: Uncaught Error: Call to undefined function mysql_query() in (path) [duplicate]

Question

I was following an online tutorial on youtube for practice, but the error (Fatal error: Uncaught Error: Call to undefined function mysql_query()) keeps on showing. even though during the demonstration of the youtuber it ran successfully. I searched matters related to this problem and added mysqli on parts of my code. I am new to php and idk how to fix this problem can you guys point out what I am doing wrong? Highly appreciated