title: What does " Mozilla/5.0" in user agent string signify?
body: <p>I am using <code>$_SERVER['HTTP_USER_AGENT']</code> to record the user's browser and operating system in use after he logs in my website, when I <code>echo</code> this it shows me these</p>
  
  <p><strong>Firefox</strong></p>
  
  <pre><code>Mozilla/5.0 (Windows NT 6.1; rv:10.0.2) Gecko/20100101 Firefox/10.0.2
  </code></pre>
  
  <p><strong>Chrome</strong></p>
  
  <pre><code>Mozilla/5.0 (Windows NT 6.1) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.162 Safari/535.19
  </code></pre>
  
  <p><strong>Safari</strong></p>
  
  <pre><code>Mozilla/5.0 (Windows NT 6.1) AppleWebKit/534.50 (KHTML, like Gecko) Version/5.1 Safari/534.50
  </code></pre>
  
  <p><strong>Opera</strong></p>
  
  <pre><code>Opera/9.80 (Windows NT 6.1; U; en) Presto/2.9.168 Version/11.50
  </code></pre>
  
  <p><strong>IE</strong></p>
  
  <pre><code>Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
  </code></pre>
  
  <p>I'm interested to know why <strong>Mozilla/5.0</strong> is prefixed when I <code>echo</code> it in IE, Safari And Chrome?</p>