$loginurl = "https://login.salesforce.com/services/oauth2/token";

$params = "grant_type=password"
. "&client_id=" . CLIENT_ID
. "&client_secret=" . CLIENT_SECRET
. "&username=" . USER_NAME
. "&password=" . PASSWORD;

$curl = curl_init($loginurl);
curl_setopt($curl, CURLOPT_HEADER, false);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, $params);

$json_response = curl_exec($curl);

$status = curl_getinfo($curl, CURLINFO_HTTP_CODE);

if ( $status != 200 ) {
    die("Error: call to URL failed with status $status, response $json_response, curl_error " . curl_error($curl) . ", curl_errno " . curl_errno($curl));
}

curl_close($curl);

echo $json_response;