php - What do commas mean in a variable declaration?

I've found this in includes/parser/Parser.php of MediaWiki PHP source:

public function replaceInternalLinks2( &$s ) {
    global $wgExtraInterlanguageLinkPrefixes;
    static $tc = false, $e1, $e1_img;
    //...
}

What is this comma delimited list? What value $tc receives?