A few different ways to use URL information in WordPress themes, from a mini-tutorial here: http://zoerooney.com/blog/tutorials/using-the-referring-page-in-wordpress/

jquery-hash-filtering.js

jQuery(document).ready(function($){

  // Read URL and determine if it contains "free"
	if(window.location.href.indexOf("free") > -1 ) {
		// If it does, filter the list
		$.filtrify("patterns-list", "filter", {
			close : true, // let's not have any drop downs open
			query : {
			           "cost" : ["free"] // pre-set the filter to only show free patterns
			       },
			callback : writeLegend // and display the legend
		});
	} else {
		// If it doesn't, show the regular full list of patterns, unfiltered
		$.filtrify("patterns-list", "filter", {
			close : true, // still closed drop downs
			callback : writeLegend // still with legend functionality
		});
	}
	$('li.three a').click(function(){
		// Force the page to reload when you are on Patterns 
		// and click "Free Patterns" via the menu
		// since usually hash URLs on the same page don't reload
		// (There may be a better way to do this, but this works)
	    window.location.hash = 'free';
	    window.location.reload();
	  });
	
	// Most of this part is Filtrify's legend code
	// http://luis-almeida.github.io/filtrify/legend.html
	function writeLegend ( query, match, mismatch ) {
       if ( !mismatch.length ) {
           $("#legend").html("");
       } else {
           var category, tags, i, tag, legend = "<h4>Currently filtered by:</h4>";
           for ( category in query ) {
               tags = query[category];
               if ( tags.length ) {
                   legend += "";
                   for ( i = 0; i < tags.length; i++ ) {
                       tag = tags[i];
                       legend += "" + tag + "&nbsp;&nbsp;&nbsp;&nbsp;";
                   };
                   legend += "";
               };
           };
           $("#legend").html( legend );
       };
       
       // I do want to check if the user has filtered by "Free" in the drop down
       // I found it easiest to do this using my legend
       // This way it's triggered every time the legend changes
       if ($('#legend:contains("free")').length) {
       		// if the legend has "free" in it, add the hash
       		// and change the page title
       		location.hash = 'free';
       		$('.page-title').html('Free Patterns').addClass('free');
       } else { 
       		// otherwise, remove the hash
       		// and change the title back to normal
       		location.hash = '';
       		$('.page-title').html('Patterns').removeClass('free');
       }
   };
  
});

referrer-contains-string.php

<div id="back-to-blog">
  <?php
    // assign the referring page URL to the variable $url
    $url = htmlspecialchars($_SERVER['HTTP_REFERER']);
	
    // check if the URL contains /blog/ using a double negative
    if (strpos($url,'/blog/') !== false) {
    
      // if it does, print the link to that URL
		  echo "<a href='$url'>BACK TO THE BLOG</a>";
	  
    } 
  ?>
</div>

url-is-specific.php

<?php
  // use a WordPress tag http://codex.wordpress.org/Function_Reference/wp_get_referer
  // to assign the referring URL to the variable $referer
  $referer = wp_get_referer();
  
  // check if the URL is a specific one
  if ( $referer == "http://lovetaza.com/once-upon-a-time/" ) {

    // if it is, do something
    
  } else {
  
    // if it isn't, do something else
  
  }

?>