Deprecations in PHP 8.1–8.4

Later 8.x versions deprecate patterns common in legacy code. 8.1: passing null to non-nullable internal function parameters (strlen(null)), Serializable without __serialize. 8.2: dynamic properties (assigning undeclared properties) unless the class has #[AllowDynamicProperties], ${var} string interpolation. 8.4: implicitly nullable parameters (Type $x = null must become ?Type $x = null). Deprecations become errors in PHP 9, so fix them now.

<?php
class Legacy {
    public function __construct() {
        $this->cache = [];        // 8.2: Deprecated: Creation of dynamic property
    }
}
// Fix: declare the property
class Fixed {
    private array $cache = [];
}

strlen($maybeNull);               // 8.1: Deprecated when $maybeNull is null
strlen($maybeNull ?? '');         // fix

function find(Logger $logger = null) {}     // 8.4: implicitly nullable — deprecated
function find2(?Logger $logger = null) {}   // fix

Common pitfalls

• Silencing deprecations instead of logging and fixing them.
• Adding #[AllowDynamicProperties] everywhere rather than declaring properties.
• Not testing with error_reporting=E_ALL in CI.

Key points

• Declare properties; avoid dynamic ones
• Handle null before calling internal functions
• Fix deprecations before PHP 9

Further reading: PHP manual: PHP 8.2 deprecations