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Named parameters, introduced in PHP 8.0, allow developers to pass arguments to a function by specifying the target parameter’s name rather than relying on its positional order in the function signature. This mechanism decouples the argument list from the function declaration order and allows explicit assignment of values to specific parameters.

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Syntax

The syntax utilizes the parameter name (omitting the $ prefix) followed by a colon : and the value being passed. 
function_name(parameterName: $value);

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Core Mechanics and Rules

1. Order Independence Arguments can be passed in any arbitrary sequence, provided all required parameters are fulfilled by the time the function executes. 
function setCoordinates($x, $y, $z) {}

// Valid: Order is resolved at runtime
setCoordinates(y: 20, z: 30, x: 10);
2. Skipping Optional Parameters Named parameters allow the caller to skip parameters that possess default values without needing to explicitly pass those defaults for preceding optional parameters. 
function buildQuery($table, $limit = 10, $offset = 0, $orderBy = 'id') {}

// Valid: $limit and $offset use defaults, $orderBy is explicitly assigned
buildQuery(table: 'users', orderBy: 'created_at');
3. Mixing Positional and Named Arguments Positional and named arguments can be combined within a single function call. However, positional arguments must strictly precede named arguments. Once a named argument is declared in the argument list, all subsequent arguments must also be named. 
function process($a, $b, $c) {}

// Valid: Positional precedes named
process(1, c: 3, b: 2);

// Fatal Error: Cannot use positional argument after named argument
// process(1, b: 2, 3);
4. Overwriting Arguments Passing a value to the same parameter multiple times—either via multiple named arguments or a combination of positional and named arguments—results in a fatal error. 
// Fatal Error: Named parameter $a overwrites previous argument
process(1, a: 2, b: 3, c: 4);
5. Unknown Parameters Passing a named argument that does not exist in the target function’s signature throws an Error exception. 
// Fatal Error: Unknown named parameter $d
process(a: 1, b: 2, c: 3, d: 4);

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Advanced Behaviors

Argument Unpacking with Associative Arrays The splat operator (...) can unpack associative arrays into named parameters. The array keys are evaluated as the parameter names. 
$args = [
    'c' => 3,
    'a' => 1,
    'b' => 2
];

// Unpacks to process(c: 3, a: 1, b: 2)
process(...$args);
Interaction with Variadic Functions When named arguments are passed to a variadic function (a function utilizing ...$args in its signature), the named arguments that do not match specific declared parameters are collected into the variadic array. The parameter names are preserved as the string keys of that array. 
function collect($a, ...$rest) {
    var_dump($rest);
}

collect(a: 1, b: 2, c: 3);
// Output for $rest: ['b' => 2, 'c' => 3]
Inheritance and Contract Implications Because parameter names are evaluated at runtime, they form a strict part of the class API contract. If a child class overrides a method and alters the parameter names, calls utilizing the parent’s parameter names will result in a runtime error, violating the Liskov Substitution Principle. 
class ParentClass {
    public function execute($query) {}
}

class ChildClass extends ParentClass {
    // Signature violation if called via named parameters
    public function execute($statement) {} 
}

$obj = new ChildClass();
// Fatal Error: Unknown named parameter $query
$obj->execute(query: 'SELECT *');
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