In the PHProjekt module "contacts_view.php" I wanted to sort the contacts list by "firma" and then by "nachname".
I changed all instances of 
$sort = "nachname"; 
to 
$sort = "firma, nachname"; (note the space between arguments)
When modifying or creating a contact, and then returning to the contacts list I get the error message:
"You have an error in your SQL syntax near 'ASC' at line 1"
I then removed the space like this:
$sort = "firma,nachname"; (no space)
This looks like a MySQL problem, but is it possible that PHP and/or PHProjekt handles the space character to cause the error?