A function is a named block of code that performs a specific task, possibly acting upon a set of values given to it, or parameters, and possibly returning a single value. Functions save on compile time—no matter how many times you call them, functions are compiled only once for the page. They also improve reliability by allowing you to fix any bugs in one place, rather than everywhere you perform a task, and they improve readability by isolating code that performs specific tasks
Calling a Function in php
Functions in a PHP program can be built-in (or, by being in an extension, effectively built-in) or user-defined. Regardless of their source, all functions are evaluated in the same way:
$someValue = function_name( [ parameter, … ] );
some examples of functions:
// strlen() is a built-in function that returns the length of a string
$length = strlen("PHP"); // $length is now 3
// sin() and asin() are the sine and arcsine math functions
$result = sin(asin(1)); // $result is the sine of arcsin(1), or 1.0
// unlink() deletes a file
$result = unlink("functions.txt"); // false if unsuccessful
Defining a Function
To define a function, use the following syntax:
function [&] function_name([parameter[, ...]])
{
statement list
}
String concatenation function in php
function strcat($left, $right)
{
$combinedString = $left . $right;
return $combinedString;
}
You can nest function declarations, but with limited effect. Nested declarations do not limit the visibility of the inner-defined function, which may be called from anywhere in your program. The inner function does not automatically get the outer function’s arguments. And, finally, the inner function cannot be called until the outer function has been called, and also cannot be called from code parsed after the outer function:
function outer ($a)
{
function inner ($b)
{
echo “there $b”;
}
echo “$a, hello “;
}
// outputs “well, hello there reader”
outer(“well”);
inner(“reader”);
Function Parameters
Functions can expect, by declaring them in the function definition, an arbitrary number of arguments. There are two different ways to pass parameters to a function. The first, and more common, is by value. The other is by reference.
Passing Parameters by Value
In most cases, you pass parameters by value. The argument is any valid expression. That expression is evaluated, and the resulting value is assigned to the appropriate variable in the function. In all of the examples so far, we’ve been passing arguments by value
Passing Parameters by Reference
Passing by reference allows you to override the normal scoping rules and give a function direct access to a variable. To be passed by reference, the argument must be a variable; you indicate that a particular argument of a function will be passed by reference by preceding the variable name in the parameter list with an ampersand (&)
<?php
function doubler(&$value)
{
$value = $value << 1;
}
$a = 3;
doubler($a);
echo $a;
Because the function’s $value parameter is passed by reference, the actual value of $a, rather than a copy of that value, is modified by the function. Before, we had to return the doubled value, but now we change the caller’s variable to be the doubled value