A constant is a value that cannot be modified throughout the execution of a program. Constant are declare by using the define() function.Its prototype follows:

boolean define(string name, mixed value)Example
define("PI",3.14159);
echo PI; //returns 3.14159

PHP offers four function for including files into applications. These are:

include(), require(), include_once(), require_once().

We can create input fields with same name for "name" attribute with squire bracket at the end of the name of the name attribute, It passes data as an array to PHP super global. 
For instance :

<input name="MyArray[]" />  
<input name="MyArray[]" />  
<input name="MyArray[]" />

By reference: When we pass an argument by reference, we pass a pointer to the value in memory. The function operates on the argument. When a function changes the value of an argument passed by reference,the original value changes.By value: When we pass an argument by value, we pass a copy of the value in memory. The function operates on the copy. This means that when a function changes the value of an argument passed by value, the effect is local to that function; the copy changes but the original value in memory is not affected.

Each array consists of two components:  key and a value. Keys can be numerical or associative. Numerical key use numeric value and associative key use string value.