Yahoo (or Google) Geocoding throws an error.

Fatal error: Call to undefined function: simplexml_load_string() in
/PathToCivicspace/modules/civicrm/CRM/Utils/Geocode/Yahoo.php
on line 109

Yahoo and Google geocoding require PHP 5 for the simplexml function. Most likely installations exhibiting this error are using PHP 4.

If you are using PHP 5 and still get this error, check to ensure that simplexml is enabled.

To check: Do a phpinfo( ); on a web page and see if you get php-5.x.x as your php version. You can do this by loading a file whose content is:

<?php
phpinfo( );
?>

in your web browser

If the above shows you are running php5, search for SimpleXML on the page loaded above. You should see something like:

SimpleXML
Simplexml support   enabled
Revision    $Revision: 1.151.2.22.2.26 $
Schema support    enabled

If you don't, most likely simplexml is not compiled into your version of php5