PHP

Write solid PHP avoiding type juggling traps, array quirks, and common security pitfalls.

Quick Reference

Topic	File
Loose typing, ==, ===, type juggling, strict_types	types.md
Associative arrays, iteration, array functions	arrays.md
Traits, interfaces, visibility, late static binding	oop.md
Encoding, interpolation, heredoc, regex	strings.md
Exceptions, error handling, @ operator	errors.md
SQL injection, XSS, CSRF, input validation	security.md
PHP 8+ features, attributes, named args, match	modern.md

Critical Rules

• == coerces types: "0" == false is true — always use === for strict comparison
• in_array($val, $arr) uses loose comparison — pass true as third param for strict
• strpos() returns 0 for match at start — use === false not !strpos()
• Never concatenate SQL — use prepared statements with PDO
• htmlspecialchars($s, ENT_QUOTES) all output — prevents XSS
• isset() returns false for null — use array_key_exists() to check key exists
• foreach ($arr as &$val) — unset $val after loop or last ref persists
• static:: late binding vs self:: early binding — static respects overrides
• @ suppresses errors — avoid, makes debugging impossible
• Catch Throwable for both Error and Exception — PHP 7+
• declare(strict_types=1) per file — enables strict type checking
• strlen() counts bytes — use mb_strlen() for UTF-8 character count
• Objects pass by reference-like handle — clone explicitly with clone $obj
• array_merge() reindexes numeric keys — use + operator to preserve keys