Nullable Types (?Type)

Also Known As

TL;DR

Explanation

Nullable types (PHP 7.1+) prefix a type declaration with ? to indicate the value may be null in addition to the declared type. function find(?int $id): ?User accepts null for $id and may return a User or null. This is equivalent to the union type int|null and User|null introduced in PHP 8.0. Nullable types made it practical to type-hint optional parameters and nullable return values without resorting to mixed. Combine with the null coalescing operator (??) and nullsafe operator (?->) for clean null handling.

Common Misconception

Why It Matters

Common Mistakes

• Using ?Type when null should actually be forbidden — nullable types are sometimes overused to avoid validation.
• Forgetting that nullable return types require explicit return null statements — returning nothing returns null implicitly but ?Type requires it to be intentional.
• Not using the nullsafe operator (?->) when chaining calls on nullable return values.
• Returning null from constructors — constructors cannot be nullable; throw an exception instead.

Code Examples

// Nullable parameter used where null should be invalid:
function setAge(?int $age): void {
    $this->age = $age; // Allows null — but a person must have an age
}

// If null means 'unknown', model it explicitly:
function setAge(int $age): void { // Enforce valid age
    if ($age < 0 || $age > 150) throw new InvalidArgumentException();
    $this->age = $age;
}

// ?Type is shorthand for Type|null
function findUser(int \$id): ?User {
    return \$this->db->find(\$id); // null if not found
}

// Nullable parameter with default
function greet(?string \$name = null): string {
    return 'Hello, ' . (\$name ?? 'Guest');
}

// Typed nullable property (PHP 7.4+)
class Order {
    public ?\DateTimeImmutable \$shippedAt = null; // not shipped yet
}

// PHP 8.0+ — use union type for explicit null
function process(int|null \$value): void {}
// Equivalent to: function process(?int \$value): void {}

References

• ↗ https://www.php.net/manual/en/migration71.new-features.php#migration71.new-features.nullable-types

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