How to fix "Optional parameter declared before required" in PHP

Deprecated: Optional parameter $limit declared before required parameter $type is implicitly treated as a required parameter in /home/user/public_html/wp-content/plugins/my-plugin/includes/query.php on line 12

Common causes

• Parameters were reordered or a new required parameter was added at the end
• Old code written for PHP 5 or 7, where this was silently allowed
• Copying signatures from older WordPress or framework code
• Using = null for a nullable type instead of the ?Type syntax

How to fix it

1. Open the function in the message. Go to the file and line shown. Note which optional parameter appears before which required one.
2. Move required parameters first. Reorder the signature so all required parameters come before optional ones, for example function get($type, $limit = 10). Update every call to match the new order.
3. Or remove the useless default. If you cannot change call sites safely, delete the default value, for example function get($limit, $type). This does not change behaviour because the default was never used.
4. Use ?Type for nullable parameters. Write ?Foo $foo instead of Foo $foo = null when the parameter is required but may be null. Note that PHP 8.4 also deprecates implicit nullable types (Foo $foo = null), so ?Foo is the safe form.
5. Update third-party code. If the file is inside a plugin, theme or vendor folder, update that package rather than editing it, so your fix is not lost on the next update.

Before and after reordering parameters

// Deprecated
function fetchPosts($limit = 10, $type) { }

// Fixed: required first
function fetchPosts($type, $limit = 10) { }

// Required but nullable
function send(?Logger $logger, string $msg) { }

How to stop it happening again

• Always put required parameters before optional ones
• Use named arguments in PHP 8 instead of long optional lists
• Test with deprecation notices visible before upgrading PHP
• Run a PHP compatibility scan on legacy code