PHP - Variable Defined and Variable Scope

 PHP - Variable Defined

• A variable is a placeholder to certain information.
• Naming Convention -- Variable start with a $, are case sensitive, and contain only alphanumeric characters and underscore(s).

• Tips: Traditionally, programmers have started their variable with a lowercase string indicating the type followed by a descriptive name for the variable (e.g., $intCounter)

• Tips: Always use meaningful variable names so you don't have remember that $a and $b is

• Tips: Define variable before using them

Example

#Legal variable names:
$myvar = "Ram"
$my_var = "Ram"
$_my_var = "Ram"
$myVar = "Ram"
$MYVAR = "Ram"
$myvar2 = "Ram"

#Illegal variable names:
$2myvar = "Ram"
$my-var = "Ram"
$my var = "Ram"

Language Comparison: Variable 

PHP Variables Scope

In PHP, variables can be declared anywhere in the script.

The scope of a variable is the part of the script where the variable can be referenced/used.

PHP has three different variable scopes:

• local
• global
• static

Global and Local Scope

A variable declared outside a function has a GLOBAL SCOPE and can only be accessed outside a function:

Example

Variable with global scope:

<?php
$x = 5; // global scope

function myTest() {
  // using x inside this function will generate an error
  echo "<p>Variable x inside function is: $x</p>";
}
myTest();

echo "<p>Variable x outside function is: $x</p>";
?>

A variable declared within a function has a LOCAL SCOPE and can only be accessed within that function:

Example

Variable with local scope:

<?php
function myTest() {
  $x = 5; // local scope
  echo "<p>Variable x inside function is: $x</p>";
}
myTest();

// using x outside the function will generate an error
echo "<p>Variable x outside function is: $x</p>";
?>

PHP The global Keyword

The global keyword is used to access a global variable from within a function.

To do this, use the global keyword before the variables (inside the function):

Example

<?php
$x = 5;
$y = 10;

function myTest() {
  global $x, $y;
  $y = $x + $y;
}

myTest();
echo $y; // outputs 15
?>

PHP also stores all global variables in an array called $GLOBALS[index]. The index holds the name of the variable. This array is also accessible from within functions and can be used to update global variables directly.

The example above can be rewritten like this:

Example

<?php
$x = 5;
$y = 10;

function myTest() {
  $GLOBALS['y'] = $GLOBALS['x'] + $GLOBALS['y'];
}

myTest();
echo $y; // outputs 15
?>

PHP The static Keyword

Normally, when a function is completed/executed, all of its variables are deleted. However, sometimes we want a local variable NOT to be deleted. We need it for a further job.

To do this, use the static keyword when you first declare the variable:

Example

<?php
function myTest() {
  static $x = 0;
  echo $x;
  $x++;
}

myTest();
myTest();
myTest();
?>

Credit:- PHP.NET