curl -v https://api.sandbox.paypal.com/v1/oauth2/token
   -H "Accept: application/json"
   -H "Accept-Language: en_US"
   -u "client_id:secret"
   -d "grant_type=client_credentials"

$headers = array(
    'Accept' => 'application/json',
    'Accept-Language'=> 'en_US',
    'Authorization' => $client_id . ':' . $secret
);
$remote_post = wp_safe_remote_post( 'https://api.sandbox.paypal.com/v1/oauth2/token', array(
    'httpversion' => '1.1',
    'sslverify' => true,
    'timeout' => 60,
    'headers' => $headers,
    'body' => json_encode( array( 'grant_type' => 'client_credentials' ) ),
) );
$body = wp_remote_retrieve_body( $remote_post );

echo '<pre>' . print_r( $body, true ) . '</pre>';