need some simple help understanding a bit of pre-PHP5 code

I was turned on to nstrees from another question, and have been working through the script. It was last updated in 2005 and relied on some stuff that has since apparently been deprecated like HTTP_POST_VARS which I wasn't immediately familiar with since I've been doing PHP for only a little less than a year.

Anyways, the coding style seems weird to my rookie eyes, and I'd like a second opinion on what part of this function does:

// returns the first node that matches the '$whereclause'.
// The WHERE clause can optionally contain ORDER BY or LIMIT clauses too.
function nstGetNodeWhere ($thandle, $whereclause) {
    $noderes['l'] = 0;
    $noderes['r'] = 0;
    $res = mysql_query("SELECT * FROM ".$thandle['table']." WHERE ".$whereclause);
    if (!$res) { // problem area 1
        _prtError();
    } else {
        if ($row = mysql_fetch_array($res)) { // problem area 2
            $noderes['l'] = $row[$thandle['lvalname']];
            $noderes['r'] = $row[$thandle['rvalname']];
        }
    }
    return $noderes;
}

In the above code I marked the spots I'm not quite sure about as // problem area x, everything else is the original script.

Regarding PA1, is that just a check on whether the query was successfully run?

And PA2, NetBeans gives me a warning "Possible accidental assignment, assignments in conditions should be avoided." ... so I promptly changed that from = to == and of course broke the script, heh.

Thinking about it, I guess it's just another error check on $res, this time to make sure that some data was actually returned?

Lastly, is this bizarre PHP or am I just too green to grok it?

Thanks dude(tte)s!

Solution

Problem Area 1 is exactly what you think it is. It checks if the query ran successfully. Look at the documentation for mysql_query (since you use Netbeans you can also highlight the function name in the editor and hit F2 to get an inline popup). You're looking for the "Return Values" section. Over there it says it will return FALSE (the boolean FALSE) on any errors.

Checking

if (!$res)

is the same as checking for

if ($res == false)

Ideally you should be checking for

if ($res === false)

because it's safer since PHP is very relaxed about variables == false, but I'd rather not confuse you with that right now. If you'd like you can start by reading about strong typing.

Problem Area 2 is pretty common but will show a warning in Netbeans, yes. What it's doing is assigning a value from mysql_fetch_array() into $row, and checking if that is now true or false, while also letting you use that same value now in $row in the following code block inside the if().