Setup ProxyCommand and ProxyAgent in ssh2 - php

Question

I need to establish a connection to an remote ssh server (debug2) via php. Inside the console everything works fine for me, but I doesnt get my php code running. I get following errormessage:
ssh2_auth_pubkey_file(): Authentication failed for [Username] using public key: Callback returned error
How can I establish a proper connection to debug2 without an registration on this.example.com?
My current code look like this:
if (!function_exists("ssh2_connect")) die("function ssh2_connect doesn't exist" . PHP_EOL);
if(!($con = ssh2_connect("this.example.com", 22))){
echo "fail: unable to establish connection" . PHP_EOL;
} else {
if(!ssh2_auth_pubkey_file($con, "[Username]", $pubkeyfile, $privkeyfile)) {
echo "fail: unable to authenticate" . PHP_EOL;
//The point where the script fails
} else {
echo "okay: logged in..." . PHP_EOL;
$tunnel = ssh2_tunnel($con, 'xx.xx.xxx.17', 22);
if (!($stream = ssh2_exec($tunnel, "ls -al" ))) {
echo "fail: unable to execute command" . PHP_EOL;
} else {
stream_set_blocking($stream, true);
$data = "";
while ($buf = fread($stream,4096)) {
$data .= $buf;
}
fclose($stream);
}
}
}
My current ssh_config:
Host example
HostName this.example.com
User [Username]
ForwardAgent yes
IdentityFile ~/.ssh/id_rsa
Host debug2
ProxyCommand ssh exmaple -W xx.xx.xxx.17:22
User [otherusername2]
IdentityFile ~/.ssh/id_rsa