First-class callable syntax in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of first-class callable with method reference

What is the output of this PHP code using first-class callable syntax?

PHP

<?php
class Greeter {
    public function sayHello() {
        return "Hello!";
    }
}
$greeter = new Greeter();
$callable = $greeter->sayHello(...);
echo $callable();

A Fatal error: Uncaught Error: Call to undefined function sayHello()

B Hello!

C Object of class Closure could not be converted to string

D Hello! Hello!

💡 Hint

The syntax $object->method(...) creates a first-class callable to that method.

The code creates a first-class callable to the sayHello method of the Greeter object. Calling $callable() executes sayHello and returns "Hello!".

❓ Predict Output

intermediate

Output of first-class callable with static method

What does this PHP code output when using first-class callable syntax with a static method?

PHP

<?php
class MathOps {
    public static function square(int $x): int {
        return $x * $x;
    }
}
$squareFunc = MathOps::square(...);
echo $squareFunc(5);

A 5

B Error: Cannot use first-class callable on static method

C 25

D TypeError: Argument 1 passed must be int

💡 Hint

Static methods can be referenced with ClassName::method(...) syntax.

The first-class callable $squareFunc points to MathOps::square. Calling it with 5 returns 25.

🔧 Debug

advanced

Identify the error in first-class callable usage

What error does this PHP code produce?

PHP

<?php
function greet(string $name) {
    return "Hi, $name!";
}
$callable = greet(...);
echo $callable();

A Fatal error: Uncaught ArgumentCountError: Too few arguments to function greet()

B Hi, !

C Syntax error: unexpected '...'

D Call to undefined function greet

💡 Hint

Check how many arguments the callable is called with.

The callable expects one argument, but calling $callable() without arguments causes an ArgumentCountError.

📝 Syntax

advanced

Which option correctly creates a first-class callable for an instance method?

Choose the correct PHP syntax to create a first-class callable for the method 'run' of an object $obj.

A $callable = $obj->run(...);

B $callable = $obj->run;

C $callable = $obj->run();

D $callable = run($obj);

💡 Hint

First-class callable syntax requires three dots after the method name.

Option A uses the correct syntax $obj->method(...). Option A is invalid syntax, C calls the method immediately, and D is not valid PHP.

🚀 Application

expert

Using first-class callable in array_map

What is the output of this PHP code that uses a first-class callable with array_map?

PHP

<?php
class Converter {
    public function toUpper(string $text): string {
        return strtoupper($text);
    }
}
$converter = new Converter();
$words = ['apple', 'banana', 'cherry'];
$upperWords = array_map($converter->toUpper(...), $words);
print_r($upperWords);

A Array ( [0] => ToUpper [1] => ToUpper [2] => ToUpper )

B Array ( [0] => apple [1] => banana [2] => cherry )

C Fatal error: Uncaught TypeError: Argument 1 passed to array_map must be callable

D Array ( [0] => APPLE [1] => BANANA [2] => CHERRY )

💡 Hint

array_map applies the callable to each element of the array.

The first-class callable $converter->toUpper(...) is passed to array_map, which applies it to each word, converting them to uppercase.