PHP Cheat Sheet: Functions, Constants & Reference Guide

PHP (Hypertext Preprocessor) remains one of the most widely used server-side languages in web development, powering everything from small personal sites to large-scale platforms. Whether you are new to the language or an experienced developer who needs a quick reference guide while building, this PHP cheat sheet covers every major built-in function, constant, and language construct organized by category.

This reference guide covers PHP 8.x syntax and functions. Functions deprecated or removed in PHP 8 are flagged inline.

If you are just getting started, this guide is built for beginners and experienced developers alike. Jump to any section using the table of contents below.

Table of Contents

• PHP Syntax Quick Reference
• Variable Handling Functions
• Array Functions
• Array Constants
• String Functions
• String Constants
• Math Functions
• Math Constants
• Date/Time Functions
• Date/Time Constants
• Calendar Functions
• Calendar Constants
• Directory Functions
• Directory Constants
• File System Functions
• File System Constants
• Zip Functions
• Filter Functions
• Filter Constants
• Mail Functions
• FTP Functions
• FTP Constants
• HTTP Functions
• SimpleXML Functions
• LIBXML Functions
• LIBXML Constants
• XML Parser Functions
• XML Parser Constants
• MySQLi Functions
• MySQLi Statements
• MySQLi Result Class
• MySQLi Driver Class
• MySQLi Warning Class
• MySQLi Constants
• Misc Functions
• Misc Constants
• Object-Oriented Programming
• Error Functions
• Error Constants
• Frequently Asked Questions

PHP Syntax Quick Reference

This section covers the core language syntax every PHP developer needs. If you are working through a tutorial or building your first project, these are the constructs you will use constantly. And here's a PDF you can copy. It's the at-a-glance cheat sheet for PHP.

Variables and Data Types

In PHP, every variable starts with a dollar sign ($). Variable names are case-sensitive, must begin with a letter or underscore, and can contain only alphanumeric characters and underscores.

$name = "Jane";          // String
$age = 30;               // Integer
$price = 9.99;           // Float
$active = true;          // Boolean
$nothing = null;         // Null

A global variable declared outside a function is not automatically available inside functions. Use the global keyword to access it:

$count = 0;

function increment() {
    global $count;
    $count++;
}

Comments

// Single-line comment
# Also a single-line comment

/* 
   Multi-line comments span
   multiple lines like this
*/

Multi-line comments are useful for documenting functions, classes, and complex logic blocks.

Strings and Escape Characters

$greeting = "Hello, World!";
$multiline = "Line one\nLine two";   // \n = newline
$tab = "Column1\tColumn2";           // \t = tab
$quote = "She said \"hello\"";       // \" = escaped quote
$dollar = "Cost: \$10";              // \$ = literal dollar sign

Common escape characters in PHP double-quoted strings:

Operators

Control Flow

// if / elseif / else
if ($score >= 90) {
    echo "A";
} elseif ($score >= 80) {
    echo "B";
} else {
    echo "C";
}

// switch statement
switch ($day) {
    case "Mon":
        echo "Monday";
        break;
    case "Tue":
        echo "Tuesday";
        break;
    default:
        echo "Other day";
}

// match expression (PHP 8+)
$label = match($status) {
    1 => "Active",
    2 => "Inactive",
    default => "Unknown"
};

The switch statement evaluates a single value against multiple cases. For more concise value matching in PHP 8, use match(), which is strict (uses === comparison) and returns a value directly.

Loops

PHP supports four loop types. The foreach loop is the most common when working with arrays, iterating over each value in sequence.

// for loop
for ($i = 0; $i < 5; $i++) {
    echo $i;
}

// while loop
$i = 0;
while ($i < 5) {
    echo $i;
    $i++;
}

// do...while loop
$i = 0;
do {
    echo $i;
    $i++;
} while ($i < 5);

// foreach loop (value iteration over arrays)
$colors = ["red", "green", "blue"];
foreach ($colors as $color) {
    echo $color;
}

// foreach with key => value
$person = ["name" => "Jane", "age" => 30];
foreach ($person as $key => $value) {
    echo "$key: $value";
}

The foreach loop is the standard approach for value iteration in PHP. It works with both indexed arrays (numeric index) and associative arrays (string keys). When looping in PHP over multidimensional arrays, nest foreach loops to access inner elements.

Functions

// Basic function
function greet($name) {
    return "Hello, $name!";
}

// Default parameter value
function greet($name = "World") {
    return "Hello, $name!";
}

// Type declarations (PHP 7+)
function add(int $a, int $b): int {
    return $a + $b;
}

// Anonymous function
$square = function($n) {
    return $n * $n;
};

// Arrow function (PHP 7.4+)
$square = fn($n) => $n * $n;

Object-Oriented Syntax Overview

PHP supports full OOP including classes, interfaces, traits, and inheritance. Here is a quick reference before the detailed magic methods section below.

// Class with constructor and public function
class Animal {
    public string $name;

    public function __construct(string $name) {
        $this->name = $name;
    }

    public function speak(): string {
        return "{$this->name} makes a sound.";
    }
}

// Inheritance
class Dog extends Animal {
    public function speak(): string {
        return "{$this->name} barks.";
    }
}

// Interface
interface Runnable {
    public function run(): void;
}

// Trait
trait Logger {
    public function log(string $msg): void {
        echo "[LOG] $msg";
    }
}

// Using interface + trait
class Robot implements Runnable {
    use Logger;

    public function run(): void {
        $this->log("Running...");
    }
}

Interfaces define contracts that classes must fulfill. Traits allow horizontal code reuse across classes that do not share a parent. Inheritance lets a child class extend a parent and override its methods. Together these form the backbone of structured PHP applications.

Arrays Quick Reference

// Indexed array
$arr = ["apple", "banana", "cherry"];
echo $arr[0]; // apple

// Associative array
$person = ["name" => "Jane", "age" => 30];
echo $person["name"]; // Jane

// Multidimensional arrays
$matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
];
echo $matrix[1][2]; // 6

// Common array operations
$arr[] = "new item";              // Append
array_push($arr, "another");      // Push to end
$last = array_pop($arr);          // Remove from end
$count = count($arr);             // Count elements
sort($arr);                       // Sort ascending

User Input and Sanitization

Handling user input safely is critical in any web application. Always sanitize and validate before using input in your logic or database queries.

// GET and POST input
$name = $_GET["name"] ?? "";
$email = $_POST["email"] ?? "";

// Sanitize user input
$clean = filter_input(INPUT_POST, "email", FILTER_SANITIZE_EMAIL);

// Validate
if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
    echo "Valid email";
}

// Prevent SQL injection with prepared statements
$stmt = $mysqli->prepare("SELECT * FROM users WHERE email = ?");
$stmt->bind_param("s", $clean);
$stmt->execute();

PHP 8.x New Features (Quick Reference)

Variable Handling Functions

Variables in PHP are containers for storing data. Every variable name is preceded with a dollar sign and is case-sensitive. Variable handling functions allow you to define, inspect, and manipulate variables at runtime.

• A variable name must start with a letter or an underscore
• A variable cannot start with a number
• A variable can only contain alphanumeric characters and underscores
• $Result and $result are treated as two different variables

Array Functions

An array stores a list of values in a single variable. Values in an indexed array are accessed by a numeric index starting at 0. PHP supports three types of arrays:

• Indexed: Default format. Values accessed by numeric index.
• Associative: Use named string keys instead of a numeric index.
• Multidimensional: Arrays that contain one or more arrays, allowing matrix-style data structures.

When working with multidimensional arrays, you typically nest a foreach loop inside another to iterate through inner arrays. Array built-in functions cover everything from sorting and searching to merging and filtering.

Array Constants

String Functions

Strings are sequences of characters and one of the most commonly used data types in PHP. String built-in functions handle everything from searching and replacing to encoding and binary safe string comparison between two values at a specified offset.

String Constants

Math Functions

Math functions handle values within the range of integer and float types. Use these for calculations, rounding, random number generation, and number base conversions.

Math Constants

Date/Time Functions

Date and time functions let you retrieve, format, and manipulate dates on the server running your PHP script.

Date/Time Constants

Calendar Functions

The calendar extension provides functions for converting between different calendar formats.

Calendar Constants

Directory Functions

Directory functions retrieve information about directories and their contents.

Directory Constants

File System Functions

These functions allow you to access and manipulate the file system, including reading, writing, copying, and inspecting files.

File System Constants

Zip Functions

These functions allow you to read and manipulate ZIP archives. The ZIP extension requires libzip.

Filter Functions

Filter functions validate and sanitize data coming from external sources such as forms, APIs, and URLs. They are the recommended approach for handling user input safely in PHP.

Filter Constants

Mail Functions

The mail function allows sending email directly from a PHP script.

FTP Functions

FTP functions give your script access to file servers through the File Transfer Protocol. Use them to open connections, upload and download files, rename, delete, and retrieve file information remotely.

FTP Constants

HTTP Functions

SimpleXML Functions

SimpleXML converts an XML document into a data structure that can be iterated like arrays and objects. It is the fastest way to read and manipulate XML data in PHP when the document structure is known.

LIBXML Functions

LIBXML functions are used with SimpleXML, XSLT, and DOM functions.

LIBXML Constants

XML Parser Functions

XML parser functions create event-driven parsers for reading XML documents. They do not validate documents but enable handlers to be defined for XML events like element start/end and character data.

XML Parser Constants

MySQLi Functions

MySQLi (MySQL Improved) functions let your PHP scripts interact with MySQL database servers. Use prepared statements to protect against SQL injection when handling user input in your queries.

MySQLi Statements

MySQLi Result Class

MySQLi Driver Class

MySQLi Warning Class

MySQLi Constants

Misc Functions

These miscellaneous functions cover connection management, constants, code execution, and utility tasks. Their behavior is affected by settings in php.ini.

Misc Constants

Object-Oriented Programming Magic Methods

PHP supports full object-oriented programming including classes, inheritance, interfaces, and traits. Magic methods are special hooks that PHP calls automatically in response to certain events on an object. Every magic method starts with a double underscore.

The example below shows inheritance, a public function, and the most common magic methods in action:

class Vehicle {
    public string $make;

    public function __construct(string $make) {
        $this->make = $make;
    }

    public function describe(): string {
        return "Vehicle: {$this->make}";
    }
}

class Car extends Vehicle {
    public function describe(): string {
        return "Car: {$this->make}";
    }

    public function __toString(): string {
        return $this->describe();
    }
}

$car = new Car("Toyota");
echo $car; // Car: Toyota

Error Functions

Error functions let you control how PHP handles, reports, and logs errors. Combined with exception handlers, they give you full control over error behavior in production and development environments.

Error Constants

What to Learn Next

This PHP cheat sheet covers the full range of built-in functions and constants, but knowing the language is only part of building production applications. From here, explore PHP frameworks like Laravel and Symfony to understand how structure and conventions accelerate web development. If you are preparing for interviews, the PHP interview questions guide covers the concepts and patterns employers test most.

For those still building foundational skills, this list of the best PHP books includes options suited to every level. And if you want structured learning with video instruction, check out the best PHP certifications to find a path that fits your goals.

For complete documentation on any function and even more detail on PHP, visit php.net/manual.