Using php query with datediff

Question

I have a php application using a MSSQL database, and am having problem with the following code:

$rstmp3 = CustomQuery("select count(ToolkitItemPk) as total FROM dbo.Toolkit_ComplianceItems WHERE LastCompleted IS NOT NULL AND (DATEDIFF(day, WarningDate, getdate()) < 0) AND (DATEDIFF(day, NextDue, getdate()) AND DepotComplianceFk='".$datatmp2['DepotComplianceFk']."'");
$datatmp3 = db_fetch_array($rstmp3); 
if($datatmp3["total"]) $totalOK = $datatmp3["total"]; else $totalOK = 0;

I suspect the

(DATEDIFF(day, WarningDate, getdate()) 0) < AND (DATEDIFF(day, NextDue, getdate())

Is not the correct way to do this, as I am getting the following error:

An expression of non-boolean type specified in a context where a condition is expected, near 'AND'.

I admit to being out of my depth here, so if someone could please recommend another way of doing this then that will earn my eternal gratitude.

I did have an easier statement in my table which was:

CASE
    WHEN LastCompleted IS NULL THEN 'Missing'
    WHEN LastCompleted IS NOT NULL AND (DATEDIFF(day, NextDue, getdate()) > 0) THEN 'Overdue'
    WHEN LastCompleted IS NOT NULL AND (DATEDIFF(day, WarningDate, getdate()) > 0) AND (DATEDIFF(day, NextDue, getdate()) < 0) THEN 'Pending'
    WHEN LastCompleted IS NOT NULL AND (DATEDIFF(day, WarningDate, getdate()) < 0) AND (DATEDIFF(day, NextDue, getdate()) < 0) THEN 'OK'
    ELSE 'N/A'
END AS Status,

This would have made life simpler to query, but as it isnt a physical field within the database, the column returns as unknown.