PHP

PHP Expressions & Basics

Prerequisite

Short Tags

<?php echo "$variable"; ?>

<?= $variable; ?> // This always works in PHP 5.4 and up

Prevent HTML Injection

Use htmlentities() to prevent HTML injection.

Comments

Single-line comments: // or #
HTML comment:

Variables

Strings

Double Quotes (" "): Variables inside double quotes expand.
Single Quotes (' '): Variables inside single quotes do not expand.

$name = "John";
echo "Hello, $name!";  // Output: Hello, John!
echo 'Hello, $name!';  // Output: Hello, $name!

Variable Naming

Use $ to declare variables.
Variables are case-sensitive.
Variable names must start with a letter or an underscore (_).
Can only contain alphanumeric characters and _.

Variable Scope

Static Variables

Retain a variable’s value after a function exits using static:

function counter() {
    static $count = 0;
    $count++;
    echo $count . "\n";
}
counter(); // 1
counter(); // 2
counter(); // 3

Global Scope

A variable declared outside a function has global scope and can only be accessed outside a function.

Global Keyword

Access global variables inside a function using global:

$x = 5;
$y = 10;

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

myTest();
echo $y; // Output: 15

Alternatively, use $GLOBALS:

$x = 5;
$y = 10;

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

myTest();
echo $y; // Output: 15

Local Scope

A variable declared inside a function is local to that function.

function myFunction() {
    $x = 10;
    echo $x;
}
myFunction();
// echo $x; // Error! $x is not accessible here.

Arrays

Array Types

Key-Value Pair (Associative Arrays)

$stuff = array("name" => "John", "course" => "CS50");

Indexed Arrays

$stuff = array("John", "Michel"); // Index always starts at 0

Multi-dimensional Arrays

$product = array(
    'paper' => array(
        'copier' => 'copier and multipurpose'
    )
);

echo $product['paper']['copier']; // copier and multipurpose

Creating an Empty Array and Appending Values

$va = array();
$va[] = "Hello"; // Array([0] => "Hello")
$va[1] = "world"; // Index can be assigned manually

$vaa = array();
$vaa["name"] = "John"; // Array(["name"] => "John")

Looping Through an Array

Associative Array

$stuff = array("name" => "John", "course" => "CS50");

foreach($stuff as $key => $value){
    echo "Key: " . $key . " Value: " . $value . "\n";
}

Indexed Array Loop

$stuff = array("John", "Michel");

for($i = 0; $i < count($stuff); $i++){
    echo "Index: " . $i . " Value: " . $stuff[$i] . "\n";
}

Array Functions

Array Length `count()`

$cars = array("Volvo", "BMW", "Toyota");
echo count($cars);

`isset()`

Determine if a variable is declared and is different than null.

Null Coalescing Operator

$stuff = array("name" => "John", "course" => "CS50");

$name = $stuff['name'] ?? 'not found'; // PHP 7+

Equivalent to:

$name = isset($stuff['name']) ? $stuff['name'] : 'not found';

Sorting Arrays

Sort by Values

$values = array("b" => 3, "a" => 1, "c" => 2);
asort($values); // Sorts by values

Sort by Keys

ksort($values); // Sorts by keys

Sort (Loses Keys)

sort($values); // Drops key-value association

Control Structures

Loops

While Loop

$fuel = 10;
while ($fuel > 1) {
    echo "Broom broom";
    $fuel = $fuel - 1;
}

Do-While Loop

$fuel = 10;
do {
    echo "Broom broom";
    $fuel = $fuel - 1;
} while ($fuel > 1);

For Loop

for ($i = 0; $i < 5; $i++) {
    echo "Iteration: $i\n";
}

Foreach Loop

foreach($colors as $value){
    echo "color :" . $value;
}

Loop Control

Use break to exit a loop completely.
Use continue to skip the current iteration and proceed to the next one.

Conditional Statements

If-Else

$ans = 42;
if ($ans == 42) {
    echo "Correct";
} elseif ($ans == 52) {
    echo "It's 52";
} else {
    echo "Wrong";
}

Shortened syntax:

$ans = 42;
if ($ans == 42) echo "Correct";
elseif ($ans == 52) echo "It's 52";
else echo "Wrong";

Switch Case

$favcolor = "red";
switch ($favcolor) {
    case "red":
        echo "Your favorite color is red!";
        break;
    case "blue":
        echo "Your favorite color is blue!";
        break;
    default:
        echo "Your favorite color is neither red nor blue";
}

Increment/Decrement Operators

$x = 12;
$y = 15 + $x++;
echo "x is $x and y is $y \n"; // x is 13 and y is 27

++$x (pre-increment): Increments first, then uses the value.
$x++ (post-increment): Uses the value first, then increments.

String Concatenation

$a = 'Hello ' . 'World';
echo $a; // Hello World

Ternary Operator

$www = 123;
$msg = $www > 100 ? "Large" : "Small"; // condition ? True : False
echo $msg; // Large

Side Effect Operator (`.=`)

$out = "Hello";
$out .= " World";
echo $out; // Hello World

Type Casting

+ (math operator) converts values to numbers.
. (string operator) converts values to strings.

$value = "same" + 25; // 25 ("same" is treated as 0)

Explicit casting:

$value = (int) 9.9 + 10; // 19

Boolean conversion examples:

echo "A" . FALSE . "B"; // AB
echo "A" . TRUE . "B";  // A1B

Comparison Operators

Equality (`==`)

Compares values without considering data types.

Identity (`===`) (Strict comparison)

Compares values with data type checking.

Caution:

FALSE is treated as 0, so functions returning 0 may be misinterpreted as FALSE.
Always check function return values carefully.

Functions

Global function and block function if we didn’t pass param value it will be null. Its will be show error

Call by Value

// Function constuction
function greet($name = "world") {
    return "Hello " . $name;
}
echo greet(); // Hello world

Call by Reference

//
function greet(&$name) {
    $name = "Alice";
}
$person = "John";
greet($person);
echo $person; // Alice

Modularity

include / include_once
- Show warning
require / require_once
- Stop execution and fatal error

Database Interaction (MySQL)

PDO (PHP Data Objects) - preferred
MySQLi (MySQL Improved)

Querying

Here are some syntax rules to follow:

The SQL query must be quoted in PHP.
String values inside the SQL query must be quoted.
Numeric values must not be quoted.
The word NULL must not be quoted.

Forms

Each form input must have a unique name attribute.

<form action="welcome.php" method="post">
    Name: <input type="text" name="name"><br>
    E-mail: <input type="text" name="email"><br>
    <input type="submit">
</form>

Super Global Variables

These are always accessible:

$GLOBALS - All global variables in this variable.
$_FILES
$_ENV
$_SERVER
$_COOKIE
$_REQUEST
$_SESSION
$_POST - Use for form POST method data collection.
$_GET - Use for form GET method data collection and a tag.

Data Validation

Non-empty: strlen($var) > 0
A number: is_numeric($var)
An email address: strpos($var, '@') > 0 or filter_var($var, FILTER_VALIDATE_EMAIL) !== false

Sessions

Super global variable:

session_start();
// Set session variables
$_SESSION["favcolor"] = "green";
// Remove all session variables
session_unset();
// Destroy the session
session_destroy();

Error Handling

Die Statement

if(!file_exists("welcome.txt")) {
    die("File not found"); //if connection failed terminate connection.
} else {
    $file = fopen("welcome.txt", "r");
}

Custom Error Handling

Create an error function with minimum error level and error message required:

error_function(error_level, error_message, error_file, error_line, error_context)

Error types:

E_USER_ERROR
E_USER_WARNING
E_USER_NOTICE

Set custom error handler:

set_error_handler("customError");

Trigger error:

$test = 2;
if ($test > 1) {
    trigger_error("Value must be 1 or below");
}

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