I am having issues with Array to JSON structure in PHP to be used in Javascript

I always struggle with Array to JSON formatting. I am using a selectbox plugin from TexoTela. The plugin requires a json structure as:

{
    "ajax1": "AJAX option 1",
    "ajax2": "AJAX option 2",
    "ajax3": "AJAX option 3"
}  

My php code:

    function get_selectfield_list($col, $table)
    {
        $location_list = array();

        //create an sql string and set it to the $sql variable
        $sql = "SELECT id, $col FROM $table";

        //form the sql query and set it to the $query variable
        $query = $this->db->query($sql);

        //loop through the result_array and set the results to the $location_list array
        foreach ($query->result_array() as $row)
        {
            $value = $row['id']; //set the database table id to to $value variable
            $text = $row[$col]; //set the the $string value to the $text variable
            $location_list = array($value => $text); //form the $location_list array with the the $value and $text values
        }

        echo json_encode($location_list); //convert the $location_list array into a json object and return it.                              
    }  

My PHP code returns {"4":"chickenpen 4"}

Instead of {"4":"chickenpen 4","5":"chickenpen 5", etc....}

I think that is due to the code: $location_list = array($value => $text); Every time the foreach loops it creates a new array. How do I format the array to output all the results in the foreach instead of the last result?

-Rich