A function is a block of code which can be called from any point in a script after it has been declared. It is basically a compartmentalized PHP script designed to accomplish a single task. Furthermore, code contained within functions is ignored until the function is called from another part in the script. Functions are useful because they contribute to rapid, reliable, error-reducing coding, and increase legibility by tiding up complicated code sequences.

It is good programming practice to use functions to modularize your code and to better provide reuse. To declare a function, you simply type:

<?php    function function_name(param_1, ... , param_n)    {       statement_1;       statement_2;        ...       statement_m;       return return_value;    } ?>

We can optionally pass parameters to the functions to be known as local variable, and we can also return a result with the "return value" statement. This produces the ending of the function returning a value.

Creating a simple function

Let's create two functions that will print the string "PHP Functions" five times, but the first one will not contain parameters, and the second one will. A function parameter is nothing more than a piece of data that the function requires to execute. In above example also included code to call the function.

<?php function firstFunction()  {     for($i = 0; $i != 5; $i++)         echo "<P>PHP Functions!</P>";  } //let's add parameters to that function function secondFunction($num, $msg)  {     for($i = 0; $i != $num; $i++)         echo "<P>". $msg ."</P>"; } echo "This is before the functions is called<br>"; echo "The first function output is:"   firstFuction(5,"This is a function with parameters"); echo "The second function output is:";   secondFuction(5,"This is a function with parameters");    echo "This is after the function has been called<br>"; ?>

Next example creates a function that will calculate arithmetic mean and return a result with the "return value" statement:

<?php       function aritmetic_mean($a, $b)  {     $result = ( $a + $b ) / 2;     return $result;  }  //print the the results of calculation     echo aritmetic_mean(4,6),"<br>";  echo aritmetica_mean(3242,524543),"<br>";     ?>

Variable Scope and Lifetime

It's important to note that if you define a variable within a function, that variable is only available within that function; it cannot be referenced in another function or in the main body of your program code.  This is known as a variable's scope.  The scope of a variable defined within a function is  local to that function.

If a function needs to use a variable that is defined in the main body of the program, it must reference it using the "global" keyword, like this:

<?php  function AddingNumbers ( ) {    global $sum = 2 + 2 } $sum = 0 Addnumbers ( ) echo "2 + 2 = ".$sum ?>

While the scope of a variable defined in a function is local to that function, a variable defined in the main body of code has a global scope.  The "global" keyword tells PHP to look for a variable that has been defined outside the function.

And what about lifetime for a variable?  A variable defined within a PHP program script exists only while that script is running.  When the script ends, the variable ceases to exist.  This seems pretty obvious!  Now apply it to a function:  a variable defined within a function exists only while that function is being processed; when the function ends, the variable ceases to exist.