Built-in Scalar Types

GraphQL specification describes several built-in scalar types. In graphql-php they are exposed as static methods of GraphQL\Type\Definition\Type class:

use GraphQL\Type\Definition\Type;

// Built-in Scalar types:
Type::string();  // String type
Type::int();     // Int type
Type::float();   // Float type
Type::boolean(); // Boolean type
Type::id();      // ID type

Those methods return instances of GraphQL\Type\Definition\ScalarType (actually one of subclasses). Use them directly in type definitions, or wrap in your TypeRegistry (if you use one).

Writing Custom Scalar Types

In addition to built-in scalars, you can define your own scalar types with additional validation. Typical examples of such types are Email, Date, Url, etc.

In order to implement your own type, you must understand how scalars are presented in GraphQL. GraphQL deals with scalars in following cases:

  1. When converting internal representation of value returned by your app (e.g. stored in a database or hardcoded in the source code) to serialized representation included in the response.

  2. When converting input value passed by a client in variables along with GraphQL query to internal representation of your app.

  3. When converting input literal value hardcoded in GraphQL query (e.g. field argument value) to the internal representation of your app.

Those cases are covered by methods serialize, parseValue and parseLiteral of abstract ScalarType class respectively.

Here is an example of a simple Email type:

namespace MyApp;

use GraphQL\Error\Error;
use GraphQL\Error\InvariantViolation;
use GraphQL\Language\AST\StringValueNode;
use GraphQL\Type\Definition\ScalarType;
use GraphQL\Utils\Utils;

class EmailType extends ScalarType
    // Note: name can be omitted. In this case it will be inferred from class name 
    // (suffix "Type" will be dropped)
    public $name = 'Email';

     * Serializes an internal value to include in a response.
     * @param string $value
     * @return string
    public function serialize($value)
        // Assuming internal representation of email is always correct:
        return $value;

        // If it might be incorrect and you want to make sure that only correct values are included
        // in response - use following line instead:
        // if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
        //     throw new InvariantViolation("Could not serialize following value as email: " . Utils::printSafe($value));
        // }
        // return $this->parseValue($value);

     * Parses an externally provided value (query variable) to use as an input
     * @param mixed $value
     * @return mixed
    public function parseValue($value)
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new Error("Cannot represent following value as email: " . Utils::printSafeJson($value));
        return $value;

     * Parses an externally provided literal value (hardcoded in GraphQL query) to use as an input.
     * E.g. 
     * {
     *   user(email: "user@example.com") 
     * }
     * @param \GraphQL\Language\AST\Node $valueNode
     * @param array|null $variables
     * @return string
     * @throws Error
    public function parseLiteral($valueNode, array $variables = null)
        // Note: throwing GraphQL\Error\Error vs \UnexpectedValueException to benefit from GraphQL
        // error location in query:
        if (!$valueNode instanceof StringValueNode) {
            throw new Error('Query error: Can only parse strings got: ' . $valueNode->kind, [$valueNode]);
        if (!filter_var($valueNode->value, FILTER_VALIDATE_EMAIL)) {
            throw new Error("Not a valid email", [$valueNode]);
        return $valueNode->value;

Or with inline style:

use GraphQL\Type\Definition\CustomScalarType;

$emailType = new CustomScalarType([
    'name' => 'Email',
    'serialize' => function($value) {/* See function body above */},
    'parseValue' => function($value) {/* See function body above */},
    'parseLiteral' => function($valueNode, array $variables = null) {/* See function body above */},