PHP 8.4 is a major update of the PHP language.
It contains many new features, such as property hooks, asymmetric visibility, an updated DOM API, performance improvements, bug fixes, and general cleanup.

Property hooks

Property hooks provide support for computed properties that can natively be understood by IDEs and static analysis tools, without needing to write docblock comments that might go out of sync. Furthermore, they allow reliable pre- or post-processing of values, without needing to check whether a matching getter or setter exists in the class.

class Locale
{
    private string $languageCode;
    private string $countryCode;
    public function __construct(string $languageCode, string $countryCode)
    {
        $this->setLanguageCode($languageCode);
        $this->setCountryCode($countryCode);
    }
    public function getLanguageCode(): string
    {
        return $this->languageCode;
    }
    public function setLanguageCode(string $languageCode): void
    {
        $this->languageCode = $languageCode;
    }
    public function getCountryCode(): string
    {
        return $this->countryCode;
    }
    public function setCountryCode(string $countryCode): void
    {
        $this->countryCode = strtoupper($countryCode);
    }
    public function setCombinedCode(string $combinedCode): void
    {
        [$languageCode, $countryCode] = explode('_', $combinedCode, 2);
        $this->setLanguageCode($languageCode);
        $this->setCountryCode($countryCode);
    }
    public function getCombinedCode(): string
    {
        return \sprintf("%s_%s", $this->languageCode, $this->countryCode);
    }
}
$brazilianPortuguese = new Locale('pt', 'br');
var_dump($brazilianPortuguese->getCountryCode()); // BR
var_dump($brazilianPortuguese->getCombinedCode()); // pt_BR

class Locale
{
    public string $languageCode;
    public string $countryCode
    {
        set (string $countryCode) {
            $this->countryCode = strtoupper($countryCode);
        }
    }
    public string $combinedCode
    {
        get => \sprintf("%s_%s", $this->languageCode, $this->countryCode);
        set (string $value) {
            [$this->languageCode, $this->countryCode] = explode('_', $value, 2);
        }
    }
    public function __construct(string $languageCode, string $countryCode)
    {
        $this->languageCode = $languageCode;
        $this->countryCode = $countryCode;
    }
}
$brazilianPortuguese = new Locale('pt', 'br');
var_dump($brazilianPortuguese->countryCode); // BR
var_dump($brazilianPortuguese->combinedCode); // pt_BR

Asymmetric Visibility

The scope to write to a property may now be controlled independently from the scope to read the property, reducing the need for boilerplate getter methods to expose a property’s value without allowing modification from the outside of a class.

class PhpVersion
{
    private string $version = '8.3';
    public function getVersion(): string
    {
        return $this->version;
    }
    public function increment(): void
    {
        [$major, $minor] = explode('.', $this->version);
        $minor++;
        $this->version = "{$major}.{$minor}";
    }
}

class PhpVersion
{
    public private(set) string $version = '8.4';
    public function increment(): void
    {
        [$major, $minor] = explode('.', $this->version);
        $minor++;
        $this->version = "{$major}.{$minor}";
    }
}

`#[\Deprecated]` Attribute

The new #[\Deprecated] attribute makes PHP’s existing deprecation mechanism available to user-defined functions, methods, and class constants.

class PhpVersion
{
    /**
     * @deprecated 8.3 use PhpVersion::getVersion() instead
     */
    public function getPhpVersion(): string
    {
        return $this->getVersion();
    }
    public function getVersion(): string
    {
        return '8.3';
    }
}
$phpVersion = new PhpVersion();
// No indication that the method is deprecated.
echo $phpVersion->getPhpVersion();

class PhpVersion
{
    #[\Deprecated(
        message: "use PhpVersion::getVersion() instead",
        since: "8.4",
    )]
    public function getPhpVersion(): string
    {
        return $this->getVersion();
    }
    public function getVersion(): string
    {
        return '8.4';
    }
}
$phpVersion = new PhpVersion();
// Deprecated: Method PhpVersion::getPhpVersion() is deprecated since 8.4, use PhpVersion::getVersion() instead
echo $phpVersion->getPhpVersion();

New ext-dom features and HTML5 support

New DOM API that includes standards-compliant support for parsing HTML5 documents, fixes several long-standing compliance bugs in the behavior of the DOM functionality, and adds several functions to make working with documents more convenient.

The new DOM API is available within the Dom namespace. Documents using the new DOM API can be created using the Dom\HTMLDocument and Dom\XMLDocument classes.

$dom = new DOMDocument();
$dom->loadHTML(
    <<<'HTML'
        <main>
            <article>PHP 8.4 is a feature-rich release!</article>
            <article class="featured">PHP 8.4 adds new DOM classes that are spec-compliant, keeping the old ones for compatibility.</article>
        </main>
        HTML,
    LIBXML_NOERROR,
);
$xpath = new DOMXPath($dom);
$node = $xpath->query(".//main/article[not(following-sibling::*)]")[0];
$classes = explode(" ", $node->className); // Simplified
var_dump(in_array("featured", $classes)); // bool(true)

$dom = Dom\HTMLDocument::createFromString(
    <<<'HTML'
        <main>
            <article>PHP 8.4 is a feature-rich release!</article>
            <article class="featured">PHP 8.4 adds new DOM classes that are spec-compliant, keeping the old ones for compatibility.</article>
        </main>
        HTML,
    LIBXML_NOERROR,
);
$node = $dom->querySelector('main > article:last-child');
var_dump($node->classList->contains("featured")); // bool(true)

Object API for BCMath

New BcMath\Number object enables object-oriented usage and standard mathematical operators when working with arbitrary precision numbers.

These objects are immutable and implement the Stringable interface, so they can be used in string contexts like echo $num.

$num1 = '0.12345';
$num2 = '2';
$result = bcadd($num1, $num2, 5);
echo $result; // '2.12345'
var_dump(bccomp($num1, $num2) > 0); // false

use BcMath\Number;
$num1 = new Number('0.12345');
$num2 = new Number('2');
$result = $num1 + $num2;
echo $result; // '2.12345'
var_dump($num1 > $num2); // false

$animal = null;
foreach (['dog', 'cat', 'cow', 'duck', 'goose'] as $value) {
    if (str_starts_with($value, 'c')) {
        $animal = $value;
        break;
    }
}
var_dump($animal); // string(3) "cat"

$animal = array_find(
    ['dog', 'cat', 'cow', 'duck', 'goose'],
    static fn(string $value): bool => str_starts_with($value, 'c'),
);
var_dump($animal); // string(3) "cat"

PDO driver specific subclasses

New subclasses Pdo\Dblib, Pdo\Firebird, Pdo\MySql, Pdo\Odbc, Pdo\Pgsql, and Pdo\Sqlite of PDO are available.

$connection = new PDO(
    'sqlite:foo.db',
    $username,
    $password,
); // object(PDO)
$connection->sqliteCreateFunction(
    'prepend_php',
    static fn($string) => "PHP {$string}",
);
$connection->query('SELECT prepend_php(version) FROM php');

$connection = PDO::connect(
    'sqlite:foo.db',
    $username,
    $password,
); // object(Pdo\Sqlite)
$connection->createFunction(
    'prepend_php',
    static fn($string) => "PHP {$string}",
); // Does not exist on a mismatching driver.
$connection->query('SELECT prepend_php(version) FROM php');

`new MyClass()->method()` without parentheses

Properties and methods of a newly instantiated object can now be accessed without wrapping the new expression in parentheses.

class PhpVersion
{
    public function getVersion(): string
    {
        return 'PHP 8.3';
    }
}
var_dump((new PhpVersion())->getVersion());

class PhpVersion
{
    public function getVersion(): string
    {
        return 'PHP 8.4';
    }
}
var_dump(new PhpVersion()->getVersion());

New Classes, Interfaces, and Functions

New Lazy Objects.
New JIT implementation based on IR Framework.
New request_parse_body() function.
New bcceil(), bcdivmod(), bcfloor(), and bcround() functions.
New RoundingMode enum for round() with 4 new rounding modes TowardsZero, AwayFromZero, NegativeInfinity, and PositiveInfinity.
New DateTime::createFromTimestamp(), DateTime::getMicrosecond(), DateTime::setMicrosecond(), DateTimeImmutable::createFromTimestamp(), DateTimeImmutable::getMicrosecond(), and DateTimeImmutable::setMicrosecond() methods.
New mb_trim(), mb_ltrim(), mb_rtrim(), mb_ucfirst(), and mb_lcfirst() functions.
New pcntl_getcpu(), pcntl_getcpuaffinity(), pcntl_getqos_class(), pcntl_setns(), and pcntl_waitid() functions.
New ReflectionClassConstant::isDeprecated(), ReflectionGenerator::isClosed(), and ReflectionProperty::isDynamic() methods.
New http_get_last_response_headers(), http_clear_last_response_headers(), and fpow() functions.
New XMLReader::fromStream(), XMLReader::fromUri(), XMLReader::fromString(), XMLWriter::toStream(), XMLWriter::toUri(), and XMLWriter::toMemory() methods.
New grapheme_str_split() function.

Deprecations and backward compatibility breaks

The IMAP, OCI8, PDO_OCI, and pspell extensions have been unbundled and moved to PECL.
Implicitly nullable parameter types are now deprecated.
Using _ as a class name is now deprecated.
Raising zero to the power of a negative number is now deprecated.
Passing invalid mode to round() throws ValueError.
Class constants from extensions date, intl, pdo, reflection, spl, sqlite, xmlreader are typed now.
GMP class is now final.
MYSQLI_SET_CHARSET_DIR, MYSQLI_STMT_ATTR_PREFETCH_ROWS, MYSQLI_CURSOR_TYPE_FOR_UPDATE, MYSQLI_CURSOR_TYPE_SCROLLABLE, and MYSQLI_TYPE_INTERVAL constants have been removed.
mysqli_ping(), mysqli_kill(), mysqli_refresh() functions, mysqli::ping(), mysqli::kill(), mysqli::refresh() methods, and MYSQLI_REFRESH_* constants have been deprecated.
stream_bucket_make_writeable() and stream_bucket_new() now return an instance of StreamBucket instead of stdClass.
exit() behavioral change.
E_STRICT constant has been deprecated.
Passing E_USER_ERROR to trigger_error() is now deprecated.