__invoke for callable objects in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of __invoke method call

What is the output of this PHP code when the object is called as a function?

PHP

<?php
class Greeter {
    public function __invoke(string $name): string {
        return "Hello, $name!";
    }
}
$greet = new Greeter();
echo $greet("Alice");

A Nothing is printed

B Fatal error: Uncaught Error: Call to undefined method Greeter::__invoke()

C Hello, World!

D Hello, Alice!

💡 Hint

Remember that __invoke allows an object to be called like a function.

The __invoke method is called when an object is used as a function. Here, calling $greet('Alice') triggers __invoke with 'Alice' as argument, returning 'Hello, Alice!'.

🧠 Conceptual

intermediate

Purpose of __invoke in PHP

What is the main purpose of the __invoke magic method in PHP?

A To allow an object to be called as if it were a function

B To automatically convert an object to a string

C To initialize object properties when created

D To overload arithmetic operators for objects

💡 Hint

Think about how you can use an object like a function.

The __invoke method lets you call an object like a function, executing the code inside __invoke when you do so.

❓ Predict Output

advanced

Output with multiple __invoke parameters

What will this PHP code output?

PHP

<?php
class Calculator {
    public function __invoke(int $a, int $b): int {
        return $a * $b;
    }
}
$calc = new Calculator();
echo $calc(3, 4);

A 12

B 7

C Fatal error: Too few arguments to function Calculator::__invoke()

D 0

💡 Hint

Check how the __invoke method uses the two parameters.

The __invoke method multiplies the two arguments 3 and 4, returning 12.

🔧 Debug

advanced

Identify the error when calling an object without __invoke

What error will this PHP code produce?

PHP

<?php
class Printer {
    public function printMessage() {
        echo "Printing message";
    }
}
$p = new Printer();
echo $p();

A Nothing is printed

B Fatal error: Uncaught Error: Call to undefined method Printer::__invoke()

C Printing message

D Parse error: syntax error, unexpected '()'

💡 Hint

Check if the class has __invoke defined.

Since Printer does not have __invoke, calling $p() causes a fatal error about undefined method __invoke.

🚀 Application

expert

Using __invoke for a callable class in a callback

Which option correctly uses a class with __invoke as a callback for array_map to double each number?

PHP

<?php
class Doubler {
    public function __invoke(int $n): int {
        return $n * 2;
    }
}
$numbers = [1, 2, 3, 4];
$result = array_map( /* fill here */, $numbers);
print_r($result);

A Doubler::__invoke

B "Doubler"

C new Doubler()

D [Doubler, '__invoke']

💡 Hint

array_map expects a callable. An object with __invoke is callable.

Passing an instance of Doubler works because __invoke makes the object callable. The other options are invalid or cause errors.