system()

It returns an array consisting of files and directories found in directory or returns FALSE on error.

function fumctionName(parameters) { function body }

Type hinting gives you the ability to force parameters to be objects of certain class or to be arrays. PHP 5 introduced this feature.

We can use the memory_get_usage() function, and to get the highest amount of memory used at any point, we can use the memory_get_peak_usage() function.

When you pass an argument in the above manner or say we pass 15 and 3 directly, it is called passing by value or call by value.

You can designate certain arguments as optional by placing them at the end of the list and assigning them a default value of nothing.

The lcfirst() function converts the first character of a string to lowercase.

The ucwords() function converts the first character of each word in a string to uppercase.

Except a) others are invalid names. According to the specified regular expression ([a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*), a function name like this one is valid.