<?

function f($p = "all")
{
    if ($p == "all")
    {
        print "if condition true, p = $p";
    }
    else
    {
        print "if condition false, p = $p";
    }
    print "<br>";
}

f();
f(0);
f(1);

?>

The output of this script is:

if condition true, p = all
if condition true, p = 0
if condition false, p = 1


and the problem is actually in second row generated
by function call f(0).  In this case $p is 0 but
the condition $p == "all" is true!