please help me in understanding functions of PHP language

Answer 1

Built-in functions, or user defined functions?

Answer 2

Php is primarily server side so the most basic functions are processing data from html forms like log in pages ,managing databases if you use it with mysql.It can also generate dynamic content for a Web page. If you are already comfortable with it,you could use it to create,edit,extract and manipulate files.PHP is generally excellent for web dev.