The echo() function is a tad faster than print() function; because it returns nothing, whereas print () will return 1 if the statement is successfully output. 

The action attribute determines where to send the form-data in the form submission.

The header() function sends a raw HTTP header to a client browser.Remember that this function must be called before sending the actual out put.For example, You do not print any HTML element before using this function.
It is most frequently used to redirect request to get other file. example: 
The following code can be used for it, 
header("Location:index.php");

Ans: There are four functions are used to add file in a script . Such as:
♦ include() function
♦ include_once() function
♦ require () function
♦ require_once() function

Just use the PHP CLI program with the option -a as follows:
php -a