How To Use function In PHP

How to use PHP functions

Syntax: function functionname([arguments]) { }

Functions can be placed anywhere in a page and will be available even if
called above the actual function being created. The exception to this rule is if
the function is only defined as part of a conditional statement, and is not
available to be called until that conditional statement has been evaluated.

Function names follow the same rules as other labels in PHP. A valid function name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

Example: declare of function without arguments

<?php
function greetMsg() {
    echo "Hello PHP!";
}

greetMsg(); // call the function?>

Note: Functions need not be defined before they are referenced, except when a function is conditionally defined as shown in the two examples below.

Otherwise you will get: Fatal error: Call to undefined function greetMsg()

When a function is defined in a conditional manner such as the two examples shown. Its definition must be processed prior to being called.

Example: Conditional function

<?php

$makefoo = true;/* We can't call foo() from here
   since it doesn't exist yet,
   but we can call bar() */bar();

if ($makefoo) {
  function foo()
  {
    echo "I don't exist until program execution reaches me.\n";
  }
}/* Now we can safely call foo()
   since $makefoo evaluated to true */if ($makefoo) foo();

function bar()
{
  echo "I exist immediately upon program start.\n";
}?>

Example: Function inside function

<?phpfunction foo()
{
  function bar()
  {
    echo "I don't exist until foo() is called.\n";
  }
}/* We can't call bar() yet
   since it doesn't exist. */foo();/* Now we can call bar(),
   foo()'s processing has
   made it accessible. */bar();?>

Note: All functions and classes in PHP have the global scope - they can be called outside a function even if they were defined inside and vice versa.

PHP function with arguments:

<?php
function mySkill($skill) {
    echo "I have $skill \t skill<br>";
}

mySkill("PHP");
mySkill("HTML");
mySkill("CSS");

?>

PHP function with default arguments

<?php
function myAge($year = 30) {
    echo "I am $year \t years old <br>";
}

myAge(29);
myAge(); // will use the default value of 30myAge(32);
?>

PHP function with returning values

If you want to return a value from function, use return statement

<?php
function add($a, $b) {
    $c = $a + $b;
    return $c;
}

echo "5 + 10 = " . add(5, 10) . "<br>";
echo "7 + 13 = " . add(7, 13) . "<br>";
echo "2 + 4 = " . add(2, 4);
?>


PHP function also return a variable

<?php
function add($a, $b) {
    $c = $a + $b;
    return $c;
}

$a  = 15;
$b = 15;
$c =  add($a, $b);
echo  $c;
?>

Tips: If multiple pages will refer to the same functions,create a separate
functions.phpfile (name it whatever you like) and require()or
require_once()with pages that will need to use those functions. For speed
and file size, page specific functions should be included directly on the
necessary page.