<route url="/V1/customers/authenticate/:email/:password" method="GET">
    <service class="MagentoCustomerApiAccountManagementInterface" method="authenticate"/>
    <resources>
        <resource ref="anonymous"/>
    </resources>
</route>

$userData = array("username" => "your_username", "password" => "your_password");
    $ch = curl_init("http://localhost/magento/index.php/rest/V1/integration/admin/token");
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($userData));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Content-Lenght: " . strlen(json_encode($userData))));

    $token = curl_exec($ch);

    $ch = curl_init("http://localhost/magento/index.php/rest/V1/customers/authenticate/".
        "roni_cost@example.com". //username/email
        "/".
        "roni_cost3@example.com" //password
        );
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "GET");
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json", "Authorization: Bearer " . json_decode($token)));

    $result = curl_exec($ch);

    echo($result);