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