PHP

Constants

A constant is a name (placeholder) for a value that does not change while the script runs. Constants are usually written in CAPITAL letters so that they are easy to find in the code. A constant name must begin with a letter or an underscore – never with a number – and constant names are case-sensitive. Constants are not prefixed with $.

You define a constant with the function define(). Once defined, a constant cannot be changed or undefined.

define("CONSTANT_NAME", value);
const CONSTANT_NAME = value;      // compile-time alternative (also used in classes)

<?php
define("SUBJECT", "PHP/MySQLi");
const INSTITUTE = "Bhopal";
echo "We all are learning: " . SUBJECT . "<br>";
echo "Batch at " . INSTITUTE . "<br>";
echo "Is SUBJECT defined? " . (defined("SUBJECT") ? "Yes" : "No") . "<br>";
echo "PHP_VERSION is a built-in constant: " . PHP_VERSION . "<br>";
echo "PHP_INT_MAX = " . PHP_INT_MAX;
?>

We all are learning: PHP/MySQLi
Batch at Bhopal
Is SUBJECT defined? Yes
PHP_VERSION is a built-in constant: 8.3.6
PHP_INT_MAX = 9223372036854775807

Variables

Variables store values – text strings, numbers or arrays. Once a variable is declared it can be used again and again in the script. All variables in PHP start with a $ sign.

$var_name = value;

New PHP programmers often forget the $ sign at the beginning of the variable – then it does not work. Let us create a variable containing a string and a variable containing a number:

<?php
$txt = "Hello World!";
$x = 16;
$price = 99.50;
echo $txt . "<br>";
echo "x = $x and price = $price<br>";
$x = $x + 4;            // a variable can be changed
echo "Now x = $x";
?>

Hello World!
x = 16 and price = 99.5
Now x = 20

PHP is a loosely typed language

In PHP a variable does not need to be declared before a value is assigned to it, and you do not tell PHP which data type it has. PHP automatically gives the variable the correct data type, depending on its value. In a strongly typed language (C, Java) you must declare the type and the name of a variable before using it.

Naming rules for variables

• A variable name must start with a letter or an underscore _ (after the $).
• A variable name may contain only letters, numbers and underscores (a–z, A–Z, 0–9, _).
• A variable name must not contain spaces. Write multi-word names as $my_string or $myString.
• Variable names are case-sensitive: $Employee and $employee are different variables.

Data Types

var_dump() shows the type and the value of a variable; gettype() returns the type name as a string.

<?php
$i = 42;
$f = 3.75;
$s = "PHP";
$b = true;
$a = array("red", "green");
$n = null;
echo "<pre>";
var_dump($i);
var_dump($f);
var_dump($s);
var_dump($b);
var_dump($a);
var_dump($n);
echo "gettype(\$f) = " . gettype($f) . "\n";
echo "</pre>";
?>

int(42)
float(3.75)
string(3) "PHP"
bool(true)
array(2) {
  [0]=>
  string(3) "red"
  [1]=>
  string(5) "green"
}
NULL
gettype($f) = double

Type juggling and casting

Because PHP is loosely typed, it converts values automatically when needed ("type juggling"). You can also convert explicitly with a cast – (int), (float), (string), (bool), (array).

<?php
$a = "10";            // a string
$b = 5;               // an integer
$c = $a + $b;         // string "10" is converted to a number
echo "\"10\" + 5 = $c (type " . gettype($c) . ")<br>";
echo "\"10\" . 5 = " . ($a . $b) . "<br>";   // . joins strings
echo "(int) 12.9 = " . (int) 12.9 . "<br>";
echo "(float) \"3.5kg\" = " . (float) "3.5kg" . "<br>";
echo "(bool) 0 is ";
var_dump((bool) 0);
echo "<br>(bool) \"hello\" is ";
var_dump((bool) "hello");
?>

"10" + 5 = 15 (type integer)
"10" . 5 = 105
(int) 12.9 = 12
(float) "3.5kg" = 3.5
(bool) 0 is bool(false)
(bool) "hello" is bool(true)

Variable Scope

The scope of a variable is the part of the script in which it can be used. In most cases variables are local: a variable declared inside a function cannot be accessed outside the function, and a variable declared outside a function cannot be accessed inside it. To use a global variable inside a function you must declare it global or access it through the $GLOBALS array.

<?php
$site = "phpbook";          // global scope

function showLocal() {
    $msg = "inside";        // local scope
    echo "Local msg: $msg<br>";
    echo "Global site without 'global': " . @$site . "(empty)<br>";
}

function showGlobal() {
    global $site;           // bring the global variable into the function
    echo "With global keyword: $site<br>";
    echo "With \$GLOBALS array: " . $GLOBALS['site'] . "<br>";
}

function counter() {
    static $count = 0;      // keeps its value between calls
    $count++;
    echo "counter() called $count time(s)<br>";
}

showLocal();
showGlobal();
counter(); counter(); counter();
?>

Local msg: inside
Global site without 'global': (empty)
With global keyword: phpbook
With $GLOBALS array: phpbook
counter() called 1 time(s)
counter() called 2 time(s)
counter() called 3 time(s)

• A static variable inside a function keeps its value between calls, instead of being reset each time.
• The @ operator used above suppresses the warning for the undefined local $site; normally you should not hide errors this way.

register_globals (historical)

Older PHP books mention the php.ini setting register_globals. When it was on, every form field, cookie and URL parameter automatically became a normal variable – ?name=Asha created $name. This allowed attackers to inject values into variables, so the default was switched to off in PHP 4.2, it was deprecated in 5.3 and removed in PHP 5.4. Modern PHP never creates variables from request data automatically; you always read them from the superglobal arrays below.

Superglobals – Variables Passed Between Pages

Superglobals are built-in arrays that are available in every scope – inside and outside functions – without the global keyword.

<?php
echo "Script name : " . $_SERVER['PHP_SELF'] . "<br>";
echo "Request method : " . $_SERVER['REQUEST_METHOD'] . "<br>";
echo "Query string : " . $_SERVER['QUERY_STRING'] . "<br>";
echo "City from \$_GET : " . $_GET['city'] . "<br>";
echo "PIN from \$_REQUEST : " . $_REQUEST['pin'] . "<br>";
echo "Server name : " . $_SERVER['SERVER_NAME'];
?>

Script name : /phpbook/ch04/server.php
Request method : GET
Query string : city=Bhopal&pin=462011
City from $_GET : Bhopal
PIN from $_REQUEST : 462011
Server name : localhost