Problem when I assign a variable to ssh2_sftp($connection)

Question

I'm trying to use ssh2_sftp($connection) but if i assign variable like that

$sftp = ssh2_sftp($connection)

My serveur return error 503

But if i do print_r(ssh2_sftp($connection)) i've ressource :

Resource id #137

The problem is for the rest of the code i can't do that :

$stream = fopen("ssh2.sftp://$sftp/path/to/file", 'r');