尝试以数组方式访问 null，bool，
int，float 或 resource
（例如 $null["key"]）将会抛出 notice 通知。
get_declared_classes() 函数将不再返回匿名的类，假如它们没有被实例化的话。
fn 关键词
fn 成为了保留关键词。需要特别注意，它不能再做为函数名或类名使用，但是仍然可以做为方法名和常量名存在。
<?php 标签
文件尾部的 <?php
标签（不包含空行）将会被解释成一个 PHP 头标签。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).
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).
序列化类型 o 被移除。因为它不是由 PHP
生成的，这可能会影响到之前项目中手动生成的序列化字符串。
密码哈希算法标识符现在是可空字符串，而不再是整数。
PASSWORD_DEFAULT 之前是 int 1; 现在是“2y”（在 PHP 7.4.0、7.4.1、7.4.2 中是 null）
PASSWORD_BCRYPT 之前是 int 1; 现在是 string '2y'
PASSWORD_ARGON2I 之前是 int 2; 现在是 string 'argon2i'
PASSWORD_ARGON2ID 之前是 int 3; 现在是 string 'argon2id'
应用中如果正常使用了常量 PASSWORD_DEFAULT，PASSWORD_BCRYPT，PASSWORD_ARGON2I 和 PASSWORD_ARGON2ID 将不会受到影响。
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().
如果传递了例如 "32foo" 这种格式不正确的数字，BCMath 函数现在将发出警告。和以前一样，参数将解释为 0。
现在尝试序列化 CURLFile 类将会生成异常。之前仅会在反序列化时引发。
弃用 CURLPIPE_HTTP1，并在 cURL 7.62.0 起不再支持。
弃用 curl_version() 的 $version 参数。如果传递了任何不等于默认
CURLVERSION_NOW 的值，则会引发警告并忽略参数。
Calling var_dump() or similar on a DateTime or DateTimeImmutable instance will no longer leave behind accessible properties on the object.
DateInterval 对象的比较（使用
==、< 等）现在将生成警告并始终返回
false。之前所有的 DateInterval 对象都认为相等，除非它们有属性。
The default parameter value of idn_to_ascii() and
idn_to_utf8() is now INTL_IDNA_VARIANT_UTS46
instead of the deprecated INTL_IDNA_VARIANT_2003.
The embedded server functionality has been removed. It was broken since at least PHP 7.0.
The undocumented mysqli::$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.
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.
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.
现在如果尝试对 Reflection 对象序列化将会生成异常。从未支持反射对象序列化并会引起反射对象错误。现在已明确禁止。
ReflectionClassConstant、ReflectionMethod 和 ReflectionProperty 类常量的值发生了变化。
现在对 ArrayObject 实例调用 get_object_vars()
将始终返回 ArrayObject 本身（或子类）的属性。
Previously it returned the values of the wrapped
array/object unless the ArrayObject::STD_PROP_LIST
flag was specified.
其它受到影响的操作有：
(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.
从 PHP 7.4.11 开始，为了安全考虑，接受到的 Cookie 中的 names 参数不再被 URL 编码。