Here you go, I commented each line, if you have a question don't hesitate to ask.

<?php

// The URL To open for parsing 
$url = "http://www.w3.org/QA/Tips/";

// attach the file to a variable
$rawLines = file($url);

// create an array
$lines = array();

// set the event counter to -1 to offset the array
$lineNo = -1;

// loop through each line in the file and pull out the intended lines
foreach ($rawLines as $rawLine) 
	{

	// First search for the line
	if (strstr($rawLine, '<h2 id="quality">') && strstr($rawLine, '</h2>')) 
		{

		// If it is found continue or go to the elseif statement
		strstr($rawLine, '<h2 id="quality">Quality Tips</h2>') ? 
                $parse = true : $parse = false;
		}

	// Look for the second string
	elseif ($parse == true && strstr($rawLine, '<li class="tip">')) 
		{

		// Increment the lineNo variable
		$lineNo++;

		// Add the tip string to an array by breaking apart the string. 
		$lines[$lineNo]["L"] = substr($rawLine, $start = strpos($rawLine, 
                '<li class="tip">') + 0, strpos($rawLine, '</a></li>') - $start);
		} 
	}

// Create another array to hold the individual tips
$tips = array();

// Set the count variable
$count=0;

// Now loop through each tip and go through a series of "str_replace" to clean up the string
foreach ($lines as $line)	{
	$tip = str_replace('<li class="tip">','',$line["L"]);
	$tip = str_replace('<a href="','',$tip);

	// Add the " | " to the string to later explode it for a cleaner result
	$tip = str_replace('">',' | ',$tip);

	// Now break apart the tips into the tip array using " | " as a delimiter
	$tip = explode(" | ", $tip);
	
	// Create the 2 arrays and "push" the text and links in to them
	$tip_url[]	= "http://www.w3.org/QA/Tips/" . $tip[0] . "\n";
	$tip_txt[]	= strip_tags(html_entity_decode($tip[1])) . "\n";
	$i++;
	}

// Create a random number. Seed the randomizer with the number of tips "$i"
mt_srand((double)microtime()*1000000);
$rand = mt_rand(0, $i);

// Return a random tip
print "<a href=\"" . $tip_url[$rand] . "\">" . $tip_txt[$rand] . "</a>";

?>

That is the end of the code snippet. Now you have a script that will display a random tip from the w3c site.