first, check to see your site is online. if its not, then do whatever you need to do to get it back online. google webmaster tools sometimes send notifications like that telling you it cannot access the site. it can be due to temporary downtime or downtime due to server update, etc. if the site is up, i wouldn't worry about the notification
There may be many reasons why you'd want to detect Googlebot as the User Agent. These can be left to your imagination [1] Cut and paste the following code anywhere in the <body> of your document: if(strstr(strtolower($_SERVER['HTTP_USER_AGENT']), "googlebot")) { // what to do } [2] Replace with your content Replace "// what to do" with whatever you want to happen. Tip: To make it HTML Do this; if(strstr(strtolower($_SERVER['HTTP_USER_AGENT']), "googlebot")) {?> <h1>Put your HTML here</h1> <p>anything you'd normally do</p> <?php } Let's break apart each piece of the PHP. if(condition){} - this is just a simple function that says "if x is true, do y". Now we'll work from the inside of the nested statement out. ‘HTTP_USER_AGENT' - this extracts a browser specific ID string $_SERVER - this is an array with information such as headers, paths, and script locations, which is created by the web server strtolower - returns string with all alphabetic characters converted to lowercase. strstr - returns part of haystack string starting from and including the first occurrence of needle to the end of haystack { // what to do } forward slashes // are just used to make a comment. All we're saying here, is to put whatever you want to happen between the curly brackets. If you like visuals better - this is a good explanation of the pieces of code; for more information : http://www.quicksprout.com/the-advanced-guide-to-seo-chapter-1/