example.php

<?php

class exampleExternalAPI
{
       public function echoString($string){
           echo $string;
       }
}

class api {
    public $APIs;

    public function __construct($setAPI = null){
        if (isset($setAPI)){
            return $this->setAPI($setAPI);
        }
    }

public function setAPIs($setAPIs)
{
    $this->APIs = new stdClass; // clears a previous call to this method
    if (!is_array($setAPIs)) { // if not an array
        $setAPIs = array($setAPIs); // make array
    }
    foreach ($setAPIs as $setAPIType) {
        if (class_exists($setAPIType)) {
            $this->APIs->$setAPIType = new $setAPIType;
        }
    }
    return $this->APIs;
}

    public function getListOfAPIs($update = false){
        if ($update){
            $this->updateListOfAPIs();
        }
        return  $this->listOfAPIs;
    }
}

$api = new api();
$api->setAPIs('exampleExternalAPI');
$api->APIs->exampleExternalAPI->echoString('string');