The method returns ReflectionClass object of parameter type class or NULL if none.

<?php

class A {
    function b(B $c, array $d, $e) {
    }
}
class B {
}

$refl = new ReflectionClass('A');
$par = $refl->getMethod('b')->getParameters();

var_dump($par[0]->getClass()->getName());  // outputs B
var_dump($par[1]->getClass());  // note that array type outputs NULL
var_dump($par[2]->getClass());  // outputs NULL

?>

ReflectionParameter::getClass() will cause a fatal error (and trigger __autoload) if the class required by the parameter is not defined. 

Sometimes it's useful to only know the class name without needing the class to be loaded.

Here's a simple function that will retrieve only the class name without requiring the class to exist:

<?php
function getClassName(ReflectionParameter $param) {
    preg_match('/\[\s\<\w+?>\s([\w]+)/s', $param->__toString(), $matches);
    return isset($matches[1]) ? $matches[1] : null;
}
?>

For php version >=8
ReflectionParamter::getType() is the recommended way to replace the deprecated methods:

- getClass()
e.g: $name = $param->getType() && !$param->getType()->isBuiltin() 
       ? new ReflectionClass($param->getType()->getName())
       : null;

- isArray()
e.g: $isArray = $param->getType() && $param->getType()->getName() === 'array';

- isCallable()
e.g: $isCallable = $param->getType() && $param->getType()->getName() === 'callable';

This method is available in PHP 7.0 and later.

You may use this one function instead depricated

    /**
     * Get parameter class
     * @param \ReflectionParameter $parameter
     * @return \ReflectionClass|null
     */
    private function getClass(\ReflectionParameter $parameter):?\ReflectionClass
    {
        $type = $parameter->getType();
        if (!$type || $type->isBuiltin())
            return NULL;

// This line triggers autoloader!
        if(!class_exists($type->getName()))
            return NULL;

       
        return  new \ReflectionClass($type->getName());
    }

Example of how to use getClass() in conjunction with getConstructor() to build the dependencies of a class. 

private function buildDependencies(ReflectionClass $reflection)
    {
        $constructor = $reflection->getConstructor();

        if (!$constructor) {
            return [];
        }

        $params = $constructor->getParameters();

        return array_map(function ($param) {

            $className = $param->getClass();

            if (!$className) {
                throw new Exception();
            }

            $className = $param->getClass()->getName();

            return $this->make($className);
        }, $params);
    }