if ( !$this->mId ) {
	// NOTE: subclasses may initialize mId before calling this constructor!
	$this->mId = $title->getArticleID( Title::READ_LATEST );
}

if ( !$this->mId ) {
	throw new InvalidArgumentException(
		"The Title object yields no ID. Perhaps the page doesn't exist?"
	);
}

public function getArticleID( $flags = 0 ) {
	if ( $this->mNamespace < 0 ) {
		$this->mArticleID = 0;

		return $this->mArticleID;
	}

	$linkCache = MediaWikiServices::getInstance()->getLinkCache();
	if ( $flags & self::GAID_FOR_UPDATE ) {
		$oldUpdate = $linkCache->forUpdate( true );
		$linkCache->clearLink( $this );
		$this->mArticleID = $linkCache->addLinkObj( $this );
		$linkCache->forUpdate( $oldUpdate );
	} elseif ( DBAccessObjectUtils::hasFlags( $flags, self::READ_LATEST ) ) {
		$this->mArticleID = (int)$this->loadFieldFromDB( 'page_id', $flags );
	} elseif ( $this->mArticleID == -1 ) {
		$this->mArticleID = $linkCache->addLinkObj( $this );
	}

	return $this->mArticleID;
}

private function loadFieldFromDB( $field, $flags ) {
	if ( !in_array( $field, self::getSelectFields(), true ) ) {
		return false; // field does not exist
	}

	$flags |= ( $flags & self::GAID_FOR_UPDATE ) ? self::READ_LATEST : 0; // b/c
	list( $index, $options ) = DBAccessObjectUtils::getDBOptions( $flags );

	return wfGetDB( $index )->selectField(
		'page',
		$field,
		$this->pageCond(),
		__METHOD__,
		$options
	);
}

SELECT page_id
FROM page
WHERE page_namespace = 4
AND page_title = 'WIntroduction_(historical)'