PHP Cheat Sheet – Syntax, Strings, Arrays & PDO

Running PHP Lesson

`php -v`	Shows the installed PHP version
`php hello.php`	Runs a script in the terminal
`php -S localhost:8000`	Built-in development server for the current folder
`<?php phpinfo();`	Page listing your PHP settings; never leave it online

PHP runs on the server; the browser only receives the HTML it outputs.

`<?php … ?>`	PHP code lives between these tags
`echo "Hi", " there";`	Prints one or more values
`<?= $name ?>`	Short echo tag inside HTML
`// note # note /* … */`	One-line and multi-line comments
`$city = "Chennai";`	Variables start with `$`; end statements with `;`
`$City and $city`	Two different variables: names are case-sensitive
`const SITE = "Shop";`	A constant; no `$`
`define("MAX_ITEMS", 10);`	A constant defined at runtime

In a file with only PHP, leave out the closing ?> to avoid stray whitespace.

`int float string bool array null`	The main types you will use
`var_dump($x);`	Shows the type and value, e.g. int(42)
`gettype(1.5)`	→ "double" (the historical name for float)
`is_int($x) is_string($x) is_null($x)`	Type checks that return true or false
`(int) "42abc"`	→ 42 (casting)
`intdiv(7, 2)`	→ 3, while `7 / 2` → 3.5
`0.1 + 0.2 == 0.3`	→ false: floats are not exact
`false 0 0.0 "" "0" [] null`	Falsy values; note `"0"` is falsy
`declare(strict_types=1);`	First line: no automatic conversion of scalar arguments

`'Hi $name'`	Single quotes: printed exactly as written
`"Hi $name"`	Double quotes: variables are replaced
`"Total: {$order['total']}"`	Braces for array items inside a string
`$a . $b $s .= "!"`	Join strings with a dot
`strlen("Chennai")`	→ 7 (bytes; use `mb_strlen()` for Tamil text)
`strtoupper("chai")`	→ "CHAI"
`ucfirst("madurai")`	→ "Madurai"
`trim(" hi ")`	→ "hi"
`substr("Pondicherry", 0, 4)`	→ "Pond"
`str_contains($s, "pay")`	true/false; also `str_starts_with`, `str_ends_with`
`str_replace("tea", "coffee", "tea time")`	→ "coffee time"
`number_format(1234567.891, 2)`	→ "1,234,567.89"

sprintf("%.2f", 3.14159) → "3.14" formats numbers inside text.

`10 % 3 2 ** 3`	→ 1 (remainder), 8 (power)
`+= -= *= .=`	Combined assignment
`"1" == 1`	→ true (loose: converts types)
`"1" === 1`	→ false (strict: type must match too)
`!= !==`	Not equal, loose and strict
`1 <=> 2`	→ -1 (spaceship gives -1, 0 or 1)
`&& \|\| !`	And, or, not
`$name ?? "Guest"`	Fallback if null or not set
`$x ??= 9;`	Assign only if `$x` is null or not set
`$age >= 18 ? "Adult" : "Minor"`	Ternary: short if/else
`$user?->address`	Nullsafe: gives null instead of an error

Prefer === and !==; loose comparison has surprising rules.

`if ($marks >= 90) { … }`	Runs the block when the condition is true
`elseif ($marks >= 50) { … } else { … }`	More tests, then a fallback
`match ($code) { 200 => 'OK', default => '?' }`	PHP 8: strict comparison, returns a value
`404, 410 => 'Gone',`	One match arm can list several values
`UnhandledMatchError`	Thrown if no arm matches and there's no default
`switch ($day) { case 'Sun': …; break; }`	Older form: loose comparison, needs `break`
`isset($x)`	True if set and not null
`empty($x)`	True if unset or falsy
`<?php if ($ok): ?> … <?php endif; ?>`	Alternative syntax for HTML templates

`for ($i = 1; $i <= 5; $i++) { … }`	Counts with a start, test and step
`while ($n > 0) { … }`	Repeats while the condition is true
`do { … } while ($n > 0);`	Runs at least once
`foreach ($items as $item) { … }`	Each value in an array
`foreach ($prices as $name => $price)`	Each key and value
`foreach ($nums as &$n) { $n *= 2; } unset($n);`	By reference changes the array; unset afterwards
`break; continue;`	Leave the loop / skip to the next round
`break 2;`	Leaves two nested loops
`<?php foreach ($rows as $r): ?> … <?php endforeach; ?>`	Template form for HTML

`function greet(string $name): string { … }`	Typed parameter and return type
`function tax(float $amt, float $rate = 0.18)`	Default parameter value
`tax(rate: 0.05, amt: 100)`	Named arguments (PHP 8), any order
`?string $note`	Nullable: a string or null
`int\|float $n`	Union type: either type is accepted
`function logMessage(string $m): void`	Returns nothing
`function sum(int ...$nums)`	Variadic: any number of arguments as an array
`static $count = 0;`	Keeps its value between calls
`function ($x) use ($rate) { … }`	Anonymous function; `use` brings in outer variables
`fn($x) => $x * $rate`	Arrow function: one expression, reads outer variables

Functions cannot see outside variables; pass them as parameters instead of using global.

`$fruits = ["mango", "banana"];`	Indexed array, starting at 0
`$p = ["name" => "Ravi", "age" => 30];`	Associative array: key => value
`$fruits[] = "guava";`	Adds to the end
`count($fruits)`	Number of items
`in_array("mango", $fruits, true)`	Strict search; true or false
`array_key_exists("age", $p)`	True even if the value is null
`sort() rsort() asort() ksort()`	By value; reverse; keep keys; by key
`usort($rows, fn($a, $b) => $a['price'] <=> $b['price']);`	Sort with your own rule
`array_map(fn($n) => $n * 2, [1, 2, 3])`	→ [2, 4, 6]
`array_filter($nums, fn($n) => $n > 2)`	Keeps matching items (and their keys)
`implode(", ", $fruits) explode(",", $csv)`	Array to string / string to array
`[$a, $b] = [1, 2];`	Destructuring into variables

`$_GET['q'] ?? ''`	Value from the URL query string
`$_POST['email'] ?? ''`	Value from a POST form body
`$_SERVER['REQUEST_METHOD'] === 'POST'`	Was the form submitted?
`trim($_POST['name'] ?? '')`	Remove spaces before checking
`filter_var($email, FILTER_VALIDATE_EMAIL)`	The email if valid, otherwise false
`filter_input(INPUT_GET, 'page', FILTER_VALIDATE_INT)`	An int, false if invalid, null if missing
`htmlspecialchars($s, ENT_QUOTES, 'UTF-8')`	Escape user text before printing it
`header('Location: thanks.php'); exit;`	Redirect after a successful POST
`session_start();`	Call before any output to use `$_SESSION`
`bin2hex(random_bytes(32))`	A random CSRF token to store in the session
`hash_equals($_SESSION['token'], $sent)`	Timing-safe token comparison

Validate on the server every time; browser checks are easy to bypass.

`include 'header.php';`	Missing file: warning, script continues
`require 'config.php';`	Missing file: fatal error, script stops
`require_once __DIR__ . '/db.php';`	Loads once, path relative to this file
`file_get_contents('notes.txt')`	Whole file as a string
`file_put_contents('log.txt', $line, FILE_APPEND);`	Writes; the flag adds instead of replacing
`file('list.txt', FILE_IGNORE_NEW_LINES)`	Lines as an array
`file_exists('data.json')`	true or false
`json_encode($data, JSON_PRETTY_PRINT)`	PHP array to JSON text
`json_decode($json, true)`	JSON text to an associative array
`$fh = fopen('big.txt', 'r'); fgets($fh); fclose($fh);`	Read a large file line by line

`'mysql:host=localhost;dbname=shop;charset=utf8mb4'`	The DSN: driver, host, database, character set
`$pdo = new PDO($dsn, $user, $pass, $options);`	Connect to the database
`PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION`	Errors throw exceptions (the default since PHP 8)
`PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC`	Rows come back as associative arrays
`$stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?');`	Prepared statement with a placeholder
`$stmt->execute([$id]);`	Sends the value separately from the SQL
`'… WHERE email = :email'`	Named placeholder: `execute(['email' => $email])`
`$stmt->fetch()`	Next row, or false when there are none
`$stmt->fetchAll()`	All rows as an array
`$pdo->lastInsertId()`	ID of the row just inserted
`$stmt->rowCount()`	Rows changed by UPDATE or DELETE
`catch (PDOException $e)`	Handle database errors; don't show details to visitors

Never put user input straight into SQL; always use placeholders.