 <p>Maybe it won't be a full answer to your question, but here's what I've been able to find so far : there is some kind of a partial answer in the book "<strong>Extending and Embedding PHP</strong>", written by Sara Golemon (<a href="http://rads.stackoverflow.com/amzn/click/067232704X" rel="nofollow noreferrer">amazon</a> ; some parts are also available on google books).</p> <p>The relevant part <em>(a note at the top of page 56)</em> is :</p> <blockquote> <p>Ever wonder why some extensions are configured using <code>--enable-extname</code> and some are configured using <code>--with-extename</code>? Functionnaly, there is no difference between the two. In practice, however, <code>--enable</code> is meant for features that can be turned on witout requiring any third-party libraries. <code>--with</code>, by contrast, is meant for features that do have such prerequisites.</p> </blockquote> <p>So, not a single word about performance (I guess, if there is a difference, it is only a matter of "<em>loading one more file</em>" vs "<em>loading one bigger file</em>") ; but there is a technical reason behind this possibility.</p> <p>I guess this is done so PHP itself doesn't <strong>require</strong> an additionnal external library because of some extension ; using the right option allows for users to enable or disable the extension themselves, depending on whether or not they already have that external library.</p>