microtime() Accuracy

I have a question about the accuracy of the microtime() function. Given the following code --- $st = split(" ", microtime()); // A bunch of PHP code. $et = split(" ", microtime()); $ct = ($et[0] + $et[1]) - ($st[0] + $st[1]); echo "<P>Clock: <B>$ct</B><BR>\n"; I'll regularly get $ct equal to .00172234121 and the like. My question is just how accurate is that number? I really don't care about anything past the thousandths place. Can I assume that it is at least accurate to that point? I'd imagine this is highly dependent on the OS/hardware. Anyone have a rule of thumb I can use for determining how accurate this number is given a certain OS/hardware combinatio? I'm primarily interesting in Solaris 7/Sparc, Linux/Intel and Win2k/Intel. (BTW, PHP 4.0.0 no Zend optimization). Thanks in advance! -Steve.