title: Determine extension that PHP function belongs to?
body: <p>Sometime I find a function in the PHP manual that I want to use, but when I try to use it -  it's undefined. </p>
  
  <p>How can I consistently find which extension a function belongs to? It doesn't seem to be written in the PHP manual. </p>
  
  <p>For example, <code>mb_strlen</code> belongs to the <code>mbstring</code> extension. </p>
  
  <p>Given A, how do I consistently find B?</p>