PHP

## 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