PHP字符串没有连接在一起

Question

PHP代码：

    $checked = $this->input->post('checkbox');
    $status = $this->input->post('adsstatus');
    if(in_array('all', $checked)){
        $query = 'SELECT * FROM ads'; 
    }
    else{
        $finalnumber = count($checked) - 1;
        for($i = 0; $i < count($checked); $i++){
            $query = 'SELECT * FROM ads WHERE ';
            if($i == $finalnumber){
               $query .= 'ID = $checked[$i] '; 
            }
            else{
                $query .= 'ID = $checked[$i] || '; 
            }
        }
    }
    print_r($checked);
    echo $query;

输出：

Array
(
    [0] => electronics
    [1] => kitchen
    [2] => decoratives
    [3] => homedecor
    [4] => furnitures
    [5] => toys
    [6] => vehicles
)
SELECT * FROM ads WHERE ID = $checked[$i] 

变量$ query的字符串未连接在一起。检查的变量的值是7它执行我的嵌套的else条件但仍然字符串没有连接在一起。如何解决它，请帮助我，现在我是一个无助的