php.net performs a number of actions when you browse to determine what language to best serve the site in.  IP address (to resolve back to the country you originate from), your browser's preferred language setting, the last explictly requested langauge, and the specific mirror's default language are all considered.

In your case, these values combined to suggest you'd want the display in spanish.  To override this value, simply select 'English' explicitly (from a dropdown on any manual page), or goto http://www.php.net/my.php to set your preferred language with a cookie (this page will also tell you what factors were conspiring to deliver spanish as opposed to english).