Implementing Serializable without __serialize() and __unserialize()

Either only the new methods should be implemented, if no support for PHP prior to version 7.4 is provided, or both should be implemented.

Passing null to non-nullable parameters of built-in functions

Scalar types for built-in functions are nullable by default. This behaviour is deprecated to align with the behaviour of user-defined functions, where scalar types need to be marked as nullable explicitly.

Implicit incompatible float to int conversions

The implicit conversion of float to int which leads to a loss in precision is now deprecated. This affects array keys, int type declarations in coercive mode, and operators working on ints.

Calling a static element on a trait

Calling a static method, or accessing a static property directly on a trait is deprecated. Static methods and properties should only be accessed on a class using the trait.

Returning a non-array from __sleep()

Returning a value which is not an array from __sleep() now generates a diagnostic.

Returning by reference from a void function

Autovivification from false

Autovivification is the process of creating a new array when appending to a value. Autovivification is prohibited from scalar values, false however was an exception. This is now deprecated.

Note:

Autovivification from null and undefined values is still allowed:

<?php
// From undefined
$arr[] = 'some value';
$arr['doesNotExist'][] = 2;
// From null
$arr = null;
$arr[] = 2;
?>

Verifying non-string arguments

Passing a non-string argument is deprecated. In the future, the argument will be interpreted as a string instead of an ASCII codepoint. Depending on the intended behavior, the argument should either be cast to string or an explicit call to chr() should be made. All ctype_*() functions are affected.