Alias /somedir/download "/path/to/your/script/dir"
<Location /somedir/download>
  ForceType application/octet-stream
Options -ExecCGI
DirectoryIndex None
</Location>

# With this configuration, this URL:
#   http://domain.com/somedir/somescript.php
# Can be downloaded as:
#   http://domain.com/somedir/download/somescript.php
#
# Note that this causes the source PHP to be downloaded;
# NOT the output. If you need that, do it from PHP.