## Backward Incompatible Changes

### PHP Core

#### Heredoc/Nowdoc Ending Label Interpretation

Due to the introduction of flexible heredoc/nowdoc syntax, doc strings that contain the ending label inside their body may cause syntax errors or change in interpretation. For example in:

`<?php
$str = <<<FOO
abcdefg
FOO
FOO;
?>`
the indented occurrence of `FOO` did not previously have any special meaning. Now it will be interpreted as the end of the heredoc string and the following `FOO;` will cause a syntax error. This issue can always be resolved by choosing an ending label that does not occur within the contents of the string.

#### Continue Targeting Switch issues Warning

`continue` statements targeting `switch` control flow structures will now generate a warning. In PHP such `continue` statements are equivalent to `break`, while they behave as `continue 2` in other languages.

`<?php
while ($foo) {
switch ($bar) {
case "baz":
continue;
// Warning: "continue" targeting switch is equivalent to
// "break". Did you mean to use "continue 2"?
}
}
?>`

#### Strict Interpretation of Integer String Keys on ArrayAccess

Array accesses of type `$obj["123"]`, where `$obj` implements ArrayAccess and `"123"` is an integer string literal will no longer result in an implicit conversion to integer, i.e., `$obj->offsetGet("123")` will be called instead of `$obj->offsetGet(123)`. This matches existing behavior for non-literals. The behavior of arrays is not affected in any way, they continue to implicitly convert integral string keys to integers.

#### Static Properties no longer separated by Reference Assignment

In PHP, static properties are shared between inheriting classes, unless the static property is explicitly overridden in a child class. However, due to an implementation artifact it was possible to separate the static properties by assigning a reference. This loophole has been fixed.

`<?php
class Test {
public static $x = 0;
}
class Test2 extends Test { }
Test2::$x = &$x;
$x = 1;
var_dump(Test::$x, Test2::$x);
// Previously: int(0), int(1)
// Now: int(1), int(1)
?>`

#### References returned by Array and Property Accesses are immediately unwrapped

References returned by array and property accesses are now unwrapped as part of the access. This means that it is no longer possible to modify the reference between the access and the use of the accessed value:

`<?php
$arr = [1];
$ref =& $arr[0];
var_dump($arr[0] + ($arr[0] = 2));
// Previously: int(4), Now: int(3)
?>`
This makes the behavior of references and non-references consistent. Please note that reading and writing a value inside a single expression remains undefined behavior and may change again in the future.

#### Argument Unpacking of Traversables with non-Integer Keys no longer supported

Argument unpacking stopped working with Traversables with non-integer keys. The following code worked in PHP 5.6-7.2 by accident.

`<?php
function foo(...$args) {
var_dump($args);
}
function gen() {
yield 1.23 => 123;
}
foo(...gen());
?>`

#### Miscellaneous

The ext_skel utility has been completely redesigned with new options and some old options removed. This is now written in PHP and has no external dependencies.

Support for BeOS has been dropped.

Exceptions thrown due to automatic conversion of warnings into exceptions in `EH_THROW` mode (e.g. some DateTime exceptions) no longer populate error_get_last() state. As such, they now work the same way as manually thrown exceptions.

TypeError now reports wrong types as `int` and `bool` instead of `integer` and `boolean`, respectively.

Undefined variables passed to compact() will now be reported as a notice.

getimagesize() and related functions now report the mime type of BMP images as `image/bmp` instead of `image/x-ms-bmp`, since the former has been registered with the IANA (see » RFC 7903).

stream_socket_get_name() will now return IPv6 addresses wrapped in brackets. For example `"[::1]:1337"` will be returned instead of `"::1:1337"`.

### BCMath Arbitrary Precision Mathematics

All warnings thrown by BCMath functions are now using PHP's error handling. Formerly some warnings have directly been written to stderr.

bcmul() and bcpow() now return numbers with the requested scale. Formerly, the returned numbers may have omitted trailing decimal zeroes.

### IMAP, POP3 and NNTP

rsh/ssh logins are disabled by default. Use imap.enable_insecure_rsh if you want to enable them. Note that the IMAP library does not filter mailbox names before passing them to the rsh/ssh command, thus passing untrusted data to this function with rsh/ssh enabled is insecure.

### Multibyte String

Due to added support for named captures, `mb_ereg_*()` patterns using named captures will behave differently. In particular named captures will be part of matches and mb_ereg_replace() will interpret additional syntax. See Named Captures for more information.

### MySQL Improved Extension

Prepared statements now properly report the fractional seconds for `DATETIME`, `TIME` and `TIMESTAMP` columns with decimals specifier (e.g. `TIMESTAMP(6)` when using microseconds). Formerly, the fractional seconds part was simply omitted from the returned values.

### MySQL Functions (PDO_MYSQL)

Prepared statements now properly report the fractional seconds for `DATETIME`, `TIME` and `TIMESTAMP` columns with decimals specifier (e.g. `TIMESTAMP(6)` when using microseconds). Formerly, the fractional seconds part was simply omitted from the returned values. Please note that this only affects the usage of PDO_MYSQL with emulated prepares turned off (e.g. using the native preparation functionality). Statements using connections having `PDO::ATTR_EMULATE_PREPARES`=`true` (which is the default) were not affected by the bug fixed and have already been getting the proper fractional seconds values from the engine.

### Reflection

Reflection export to string now uses `int` and `bool` instead of `integer` and `boolean`, respectively.

### Standard PHP Library (SPL)

If an SPL autoloader throws an exception, following autoloaders will not be executed. Previously all autoloaders were executed and exceptions were chained.

### SimpleXML

Mathematic operations involving SimpleXML objects will now treat the text as an int or float, whichever is more appropriate. Previously values were treated as ints unconditionally.

### Incoming Cookies

As of PHP 7.3.23, the names of incoming cookies are no longer url-decoded for security reasons.

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