WordPress MU Multiple Domains Modification
From JohnBreslin
WPMU is already multi-domain capable. To allow users to choose different domains, the following small edit is required.

In wp-newblog.php, replace the line:

print '<input name="weblog_id" type="text" id="weblog_id" value="'.$weblog_id.'" maxlength="50" style="width:40%; text-align: right; font-size: 30px;" /><span style="font-size: 30px">.' . $current_site->domain . '</span><br />(<strong>Your address will be username.' . $current_site->domain . '.</strong> Must be at least 4 characters, letters and numbers only. It cannot be changed so choose carefully!)</td> </tr>';

with:

print '<input name="weblog_id" type="text" id="weblog_id" value="'.$weblog_id.'" maxlength="50" style="width:40%; text-align: right; font-size: 30px;" /><span style="font-size: 30px">.</span><select name="weblog_domain" style="width:50%; text-align: right; font-size: 30px;" /><option value="journals.ie">journals.ie</option><option value="galwaycity.com">galwaycity.com</option></select><br />(<strong>Your address will be username.domainname.com.</strong> Must be at least 4 characters, letters and numbers only. It cannot be changed so choose carefully!)</td> </tr>';

Also, after the lines:

if( substr( $domain, 0, 4 ) == 'www.' ) $domain = substr( $domain, 4 );

add the line:

$domain = $_POST[ 'weblog_domain' ];

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