php file_get_contents return always FALSE on https://graph.facebook.com/oauth/access_token? request

Since some weeks the facebook connection to my application doesn't work.

My dialog url is this one :

$dialogUrl = "https://www.facebook.com/dialog/oauth?client_id=".$my_facebook_app_id."&redirect_uri=".urlencode($my_return_url)."&state=".$my_state."&scope=read_stream,offline_access,publish_stream";

Connection dialog accept my connection and return to $my_return_url with ?state=XXX&code=YYY

When I call the file_get_contents() method on this url with $code corresponding to the code in query string :

$tokenurl = "https://graph.facebook.com/oauth/access_token?client_id=".$my_facebook_app_id."&redirect_uri=".urlencode($my_return_url)."&client_secret=".$facebook_app_secret."&code=".$code;

I have always a FALSE result !

A few month ago it was returning the access_token, but now it always return false on my website.

Note than if I load the $tokenUrl in a navigator the access_token is returned.

Thanks for your help.