This only changes the directory for PHP, the output directory stays the same. If you are trying to access images from a relative path and you use the following then it will fail to render the image:
chdir ('images');
if (file_exists('php.gif'))
{
echo '<html>';
echo '<body>';
echo '<img src="php.gif">';
echo '</body></html>';
}
//However, it is possible to use the <base> tag in the header to change the directory for the resulting HTML, as you can see however this requires you to put the full path in place.
chdir ('images');
if (file_exists('php.gif'))
{
echo '<html>';
echo '<head><base href = "http://uk.php.net/images/"></head>';
echo '<body>';
echo '<img src="php.gif">';
echo '</body></html>';
}