These are my PHP highlights from each release. PHP do such a great job moving the language forward, and documenting all these changes but I always forget all the cool new things I can use so I've documented the most interesting ones for me here.

Shortcuts: 7.2 - 7.1 - 7.0 - 5.6 - 5.5 - 5.4 - 5.3

PHP 7.2

Migration Guide

New Features

• New object type - function tweet(object $message): object [changelog]
• Sodium added as a core extension, allowing better and easier encryption/decryption [changelog]
• Argon2 added as a new hash in the password hashing API [changelog]

Extras

• The $additional_headers parameter of mail() and mb_sendmail() now also accepts an array instead of a string [changelog]

Backwards Incompatible Changes

• object is now a hard-reserved word and cannot be used as a class, trait or interface name [changelog]

Deprecations

• __autoload has been deprecated in favour of spl_autoload_register [changelog]

PHP 7.1

Migration Guide

New Features

• Nullable types - function tweet(?string $messageOverride): ?TweetResult [changelog]
• Short-hand array destructuring - [$latitude, $longitude] = $locations[42]; [changelog]
• • This can also be used in foreach: foreach ($locations as [$latitude, $longitude]) {
• iterable pseudo-type - function tweetMessages(iterable $messages) [changelog]
• Asynchronous signal handling [changelog]

pcntl_async_signals(true); // turn on async signals

pcntl_signal(SIGTERM,  function($sig) {
    die("You can't terminate me! Do you know who I am?");
});

• Catch multiple exceptions [changelog]

try {
    $tweeter->tweet($messageOverride);
} catch (BadMessage | AccountBlocked | TooLong $e) {
  Log::error('Oops, I did it again');
}

Backwards Incompatible Changes

• void and iterable are now reserved keywords and cannot be used to name classes, traits or interfaces. [changelog]
• DateTime and DateTimeImmutable now properly incorporate microseconds - new DateTime() == new DateTime(); is likely to equal false [changelog]

PHP 7.0

Migration Guide • Backwards Incompatible Changes

PHP 7.0 has the largest number of backwards incompatible changes which may require large changes to your codebase. That said, PHP 7.0 was a huge leap in functionality, speed and progress from PHP 5.6.

New Features

• Scalar type declarations - function tweet(string $message) - string, int, float, bool [changelog] (this is my favourite new feature)
• Return type declarations - function tweet(string $message): bool [changelog]
• Null coalescing operator - $_GET['message'] ?? "Come up with your own message bro"; [changelog]
• Unicode codepoint escape syntax - echo "\u{32DB}"; will echo ㋛ [changelog]

Backwards Incompatible Changes

• mysql_ functions removed [changelog]
• Many errors have been converted to exceptions and 'Error' exceptions inherit from the Error class [changelog]
• mssql_ functions removed [changelog]
• ereg_ functions removed [changelog]
• Magic quotes removed - set_magic_quotes_runtime() and magic_quotes_runtime() [changelog]
• bool, int, float, string, NULL, TRUE, FALSE cannot be used to name classes, interfaces or traits [changelog]
• preg_replace no longer supports \e PREG_REPLACE_EVAL [changelog]

Deprecations

• PHP 4 style constructors - class Tweet { function tweet() { echo "Constructor"; } [changelog]

PHP 5.6

Migration Guide

New Features

• Variadic functions and splat operator - function tweet (...$messages) - $messages = ['I try to say goodbye', 'And I choke']; tweet(...$messages); [changelog]
• php://input is now reusable! Woo! [changelog]
• __debugInfo magic method to determine what's shown when using var_dump [changelog]

PHP 5.5

Migration Guide

New Features

• Generators added [changelog]
• New password hashing API [changelog] - This is incredibly important for bringing added security to applications
• Array dereferencing - echo ['Macy', 'Gray'][1]; == Gray [changelog]
• Addition of ::class to get a class name [changelog]
• array_column added - very useful function [documentation]
• json_last_error_msg() added [documentation]

Deprecations

• mysql_* deprecated (Removed in PHP 7.0) [changelog]
• preg_replace /e modifier (Removed in PHP 7.0) [changelog]

PHP 5.4

Migration Guide

New Features

• Short array syntax - $tweets = ['Short', 'Array', 'Syntax', 'Is', 'The', 'Best']; [changelog] - This is one of my favourite features ever
• traits were added allow 'copy and paste' of methods for classes [documentation]
• <?= is always available for a quicker way of printing [changelog]

Backwards Incompatible Changes

• PHP 5.4 htmlspecialchars and htmlentities assume the charset is UTF-8 by default and will return blank if the input is not valid UTF-8 [source]
• Magic quotes has been removed requiring code changes [changelog]
• Cannot use super globals in function declarations - function tweet($_GET, $_POST) [changelog]
• New reserved keywords trait, callable, insteadof [changelog]
• session_is_registered, session_register, session_unregister have been removed [changelog]

I'll continue to update this as each new version is released (last updated 2018-08-08).