When Did PHP Get That?

5.6.3

• 64-bit pack codes

5.6.0

• Variadic functions with "..." (and related packing/unpacking with the ... operator)
• ** operator for exponentiation
• Constant scalar expressions (const FOO = 2*60*60)
• use const and use function make constants/functions importable from other namespaces
• htmlspecialchars() and friends use the INI default_charset setting as default (update to 5.4 behavior)
• php://input can be reused and $HTTP_RAW_POST_DATA can be disabled (and will be, in the future)
• File uploads can be >2GB
• Constant-time string comparison (aka timing-safe comparison) added as hash_equals
  • I am still curious: why hash_equals? What's the relation to hashing?

5.5.0

• Generators
• finally keyword for exception handling with try/catch
• Password hashing API: password_hash and related functions
• list() in foreach: foreach($nest as list($x, $y))
• PBKDF2 support in hash and openssl (hash_pbkdf2, openssl_pbkdf2)
• empty() accepts arbitrary expressions, not just lvalues
• array and string literal dereferencing: "php"[2] or [1,2,10][2]
• But is that useful?
• ::class syntax: namespace A\B { class C { }; echo C::class; /* A\B\C */ }
• OpCache extension added
• MySQL-nd supports sha256-based authentication, which has been available since MySQL 5.6.6
• curl share handles via curl_share_init and friends [added to this post on 2015-02-17]
• DEPRECATED: preg_replace's /e modifier, in favor of preg_replace_callback

5.4.0
Removed:

• safe_mode
• register_globals
• magic_quotes
• call-time pass-by-reference
• using TZ when guessing the default timezone
• Salsa10 and Salsa20 hash algorithms

Added:

• Traits
• Class::{expr} syntax
• Short array syntax: [2,3] for array(2,3)
• Binary literal syntax: 0b1101
• Closures can use $this in the body: function some_method () { return function () { return $this->protected_thing(); } }
• Using new objects without assigning them: (new Foo)->bar()
• CLI server
• session_status function
• header_register_callback function
• mysqli_result now implements Traversable: $r=$dbh->query(...); foreach ($r as $res) { ... }
• htmlspecialchars() and friends default to UTF-8 instead of ISO-8859-1 for character set.
• This would be changed to use the default_charset INI setting in 5.6.
• 5.4's documentation pointed out that default_charset could be used by sending an empty string, "", as the character set to htmlspecialchars—and said you didn't really want it.

5.3.0
Added:

• Namespaces
• Late static binding and associated 'static::method()' call syntax
• "Limited" goto support
• Closures aka anonymous functions, although they cannot make use of $this until 5.4
• __callStatic() and __invoke() magic methods, to support $obj($arg) call syntax
• 'nowdoc' and double-quoted 'heredoc' syntax
• $x=<<<'FOO' consumes text until FOO and does not do any interpolation / variable substitution on it
• $x=<<<"FOO" acts just like traditional $x=<<<FOO, both consuming text until FOO and doing interpolation / variable substitution as if it were a double-quoted string.
• Constants definable outside of classes with the const keyword, not just the define function
• Short ternary operator: a ?: b equivalent to a ? a : b
• $cls::$foo syntax for accessing static members of a class whose name is stored in a variable ($cls in this example)
• Chained exceptions: the constructor sprouted an extra parameter for a previous exception (generally should mean "the cause of this exception being constructed")
• DEPRECATED: many features that would be removed in 5.4.0, including register_globals, magic_quotes, and safe_mode.