//FirstRevision.i18n.magic.php
<?php
$magicWords = array();

$magicWords['en'] = array(
    // magic ID
    'FIRSTREVISION' => array(1, 'FIRSTREVISION'),
);


//FirstRevision.php
<?php
if (!defined('MEDIAWIKI')) {
    die();
}

$wgExtensionMessagesFiles['FirstRevisionMagic'] = __DIR__ . '/FirstRevision.i18n.magic.php';

$wgHooks['ParserGetVariableValueSwitch'][] = 'wfMyAssignAValue';
function wfMyAssignAValue(&$parser, &$cache, &$magicWordId, &$ret) {
    if ($magicWordId == 'FIRSTREVISION') {
        $ret = 'test';
    }
    return true;
}

$wgHooks['MagicWordwgVariableIDs'][] = 'wfMyDeclareVarIds';
function wfMyDeclareVarIds(&$customVariableIds) {
    $customVariableIds[] = 'FIRSTREVISION';
    return true;
}

$wgExtensionCredits['parserhook'][] = array(
    'path' => __FILE__,
    'name' => 'FirstRevision',
    'version' => '0.1',
    'author' => 'asd',
    'descriptionmsg' => 'descriptionmsg',
);