Backward Incompatible Changes

PHP Core

Array-style access of non-arrays

Trying to use values of type null, bool, int, float or resource as an array (such as $null["key"]) will now generate a notice.

get_declared_classes() function

The get_declared_classes() function no longer returns anonymous classes that have not been instantiated yet.

fn keyword

fn is now a reserved keyword. In particular, it can no longer be used as a function or class name. It can still be used as a method or class constant name.

<?php tag at end of file

<?php at the end of the file (without trailing newline) will now be interpreted as an opening PHP tag. Previously it was interpreted either as a short opening tag followed by literal php and resulted in a syntax error (with short_open_tag=1) or was interpreted as a literal <?php string (with short_open_tag=0).

Stream wrappers

When using include/require on a stream, streamWrapper::stream_set_option() will be invoked with the STREAM_OPTION_READ_BUFFER option. Custom stream wrapper implementations may need to implement the streamWrapper::stream_set_option() method to avoid a warning (always returning false is a sufficient implementation).


The o serialization format has been removed. As it is never produced by PHP, this may only break unserialization of manually crafted strings.

Password algorithm constants

Password hashing algorithm identifiers are now nullable strings rather than integers.

  • PASSWORD_DEFAULT was int 1; now is null
  • PASSWORD_BCRYPT was int 1; now is string '2y'
  • PASSWORD_ARGON2I was int 2; now is string 'argon2i'
  • PASSWORD_ARGON2ID was int 3; now is string 'argon2id'

Applications correctly using the constants PASSWORD_DEFAULT, PASSWORD_BCRYPT, PASSWORD_ARGON2I, and PASSWORD_ARGON2ID will continue to function correctly.

htmlentities() function

htmlentities() will now raise a notice (instead of a strict standards warning) if it is used with an encoding for which only basic entity substitution is supported, in which case it is equivalent to htmlspecialchars().

fread() and fwrite() function

fread() and fwrite() will now return false if the operation failed. Previously an empty string or 0 was returned. EAGAIN/EWOULDBLOCK are not considered failures.

These functions now also raise a notice on failure, such as when trying to write to a read-only file resource.

BCMath Arbitrary Precision Mathematics

BCMath functions will now warn if a non well-formed number is passed, such as "32foo". The argument will be interpreted as zero, as before.


Attempting to serialize a CURLFile class will now generate an exception. Previously the exception was only thrown on unserialization.

Using CURLPIPE_HTTP1 is deprecated, and is no longer supported as of cURL 7.62.0.

The $version parameter of curl_version() is deprecated. If any value not equal to the default CURLVERSION_NOW is passed, a warning is raised and the parameter is ignored.

Date and Time

Calling var_dump() or similar on a DateTime or DateTimeImmutable instance will no longer leave behind accessible properties on the object.

Comparison of DateInterval objects (using ==, <, and so on) will now generate a warning and always return false. Previously all DateInterval objects were considered equal, unless they had properties.


The default parameter value of idn_to_ascii() and idn_to_utf8() is now INTL_IDNA_VARIANT_UTS46 instead of the deprecated INTL_IDNA_VARIANT_2003.


The embedded server functionality has been removed. It was broken since at least PHP 7.0.

The undocumented mysqli::$stat property has been removed in favor of mysqli::stat().


The openssl_random_pseudo_bytes() function will now throw an exception in error situations, similar to random_bytes(). In particular, an Error is thrown if the number of requested bytes is less than or equal to zero, and an Exception is thrown if sufficient randomness cannot be gathered. The $crypto_strong output argument is guaranteed to always be true if the function does not throw, so explicitly checking it is not necessary.

Regular Expressions (Perl-Compatible)

When PREG_UNMATCHED_AS_NULL mode is used, trailing unmatched capturing groups will now also be set to null (or [null, -1] if offset capture is enabled). This means that the size of the $matches will always be the same.

PHP Data Objects

Attempting to serialize a PDO or PDOStatement instance will now generate an Exception rather than a PDOException, consistent with other internal classes which do not support serialization.


Reflection objects will now generate an exception if an attempt is made to serialize them. Serialization for reflection objects was never supported and resulted in corrupted reflection objects. It has been explicitly prohibited now.

Standard PHP Library (SPL)

Calling get_object_vars() on an ArrayObject instance will now always return the properties of the ArrayObject itself (or a subclass). Previously it returned the values of the wrapped array/object unless the ArrayObject::STD_PROP_LIST flag was specified.

Other affected operations are:

(array) casts are not affected. They will continue to return either the wrapped array, or the ArrayObject properties, depending on whether the ArrayObject::STD_PROP_LIST flag is used.

SplPriorityQueue::setExtractFlags() will throw an exception if zero is passed. Previously this would generate a recoverable fatal error on the next extraction operation.

ArrayObject, ArrayIterator, SplDoublyLinkedList and SplObjectStorage now support the __serialize() and __unserialize() mechanism in addition to the Serializable interface. This means that serialization payloads created on older PHP versions can still be unserialized, but new payloads created by PHP 7.4 will not be understood by older versions.


token_get_all() will now emit a T_BAD_CHARACTER token for unexpected characters instead of leaving behind holes in the token stream.

Incoming Cookies

As of PHP 7.4.11, the names of incoming cookies are no longer url-decoded for security reasons.

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User Contributed Notes 1 note

happydog at kennel17
1 month ago
Re: "The o serialization format has been removed. As it is never produced by PHP, this may only break unserialization of manually crafted strings."

This little-o serialisation format was used by PHP3 but was never generated by PH PHP4 or above.  The deserialization code still recognised it, though, for reasons of backwards-compatibility with PHP3.

However, based on a bit of investigation, it looks like this code has been broken for about 15 years, so although this is listed as a deprecation, in practice it wasn't.

See this Stack Overflow question for a really great answer, with a lot more detail about this:
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