Priyanka, Facebook has its own SDK which allow user to login & signup via facebook

Refrence URL: https://github.com/facebook/facebook-php-sdk-v4

or you can use below code to integrate facebook login

1. Paste JavaScript code in header or footer:

The below code is a JavaScript code; this function will call when user click on login fb button.

<script type="text/javascript">
window.fbAsyncInit = function()
{
    FB.init(
    {
        appId: 'set&#95;fb&#95;app&#95;Id', // App ID
        cookie: true, // enable cookies to allow the server to access the session
        xfbml: true, // parse XFBML
        oauth: true 
    });
};

function loginViaFacebook()
{
    FB.login(function(response)
    {
        if (response.authResponse)
        {
            // connected
            window.location.href = 'http://example.com/fbuserlogin'; // set your redirect url after login
        }
        else
        {
            // User cancelled login or did not fully authorize
            location.reload(true);
        }
    },
    {
        scope: 'email,public&#95;profile'
        //https://developers.facebook.com/docs/facebook-login/permissions/v2.2
    }
    );
}

(function() {
var e = document.createElement('script'); e.async = true;
e.src = document.location.protocol +
'//connect.facebook.net/en&#95;US/all.js';
document.getElementById('fb-root').appendChild(e);
}());
</script>

note: please replace _ with "_" while copying the above code.

2. Use below html code to render facebook login button.
< input type="button" value="Sign in with Facebook" onclick="javascript:loginViaFacebook();" /> < div id="fb-root">
3. Please extract attached file & call it like this:- require_once('set_absolute_path/fb/src/facebook') then, use below PHP code to get user details after authentication from Facebook.

// Set facebook app credential
        $facebook = new Facebook(array(
            'appId'  => 'set_fb_app_Id',
            'secret' => 'set_fb_secret'
        ));

        // Get User ID
        $user = $facebook->getUser();

        if ($user)
        {
            try
            {
                // Proceed knowing you have a logged in user who's authenticated.
                $user_profile = $facebook->api('/me');

                // Get profile image
                $profile_image = $facebook->api('me/picture', array('redirect'=>false, 'height'=>'125', 'type'=>'normal', 'width'=>'125'));

                // Process your logic here
            }
            catch (FacebookApiException $e)
            {
                $user = null;
            }
        }