$username = 'myusername';
$password = 'mypassword';
$curl = curl_init();

curl_setopt($curl, CURLOPT_POST, 1);
curl_setopt($curl, CURLOPT_URL, "https://api.neteller.com/v1/oauth2/token?grant_type=client_credentials");
curl_setopt($curl, CURLOPT_USERPWD, "$username:$password");
curl_setopt($curl, CURLOPT_HTTPHEADER, array("Content-Type:application/json", "Cache-Control:no-cache"));
curl_setopt($curl, CURLOPT_POSTFIELDS, array("scope"=>"default"));
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);

$serverOutput = curl_exec($curl);

echo $serverOutput;

Error: { "error": "invalid_client" }