$new_arr=array_column($tags,0,1);

$tags = array_combine($tags[0], $tags[1]);

$ret = [];
$tags = [
    [
        "taxfree",
        "房本滿五年"
    ],
    [
        "haskey",
        "隨時(shí)看房"
    ]
];
foreach($tags as $tag) {
    $ret[$tag[0]] = $tag[1];
}