<div class="col-sm-5">
    <select name="car_id" id="car_id" class="form-control">

        <?php while($car_row=@mysqli_fetch_assoc($car_result))
        {
            ?>
            <option value="<?php echo $car_row['car_id'];?>"><?php echo $car_row['car_name'];?></option>
        <?php }?>
    </select>
</div>
<div class="col-sm-5">
    <select name="km_id" id="km_id" class="form-control">
        <?php while($km_row=@mysqli_fetch_assoc($km_result))
        {
            ?>
            <option value="<?php echo $km_row['km_id'];?>"><?php echo $km_row['km'];?></option>

        <?php }?>
    </select>
</div>

<div class="col-sm-12" align="center" style="font-size: 20px;padding-top: 50px;padding-bottom: 150px;">
    <input class="btn-danger" id="search_button" name="search" onclick="service('maintenance.php')" type="button" value="بحث" />

<script>
var c_id=$('#car_id').val();
var k_id=$('#km_id').val();

var parameter='car_id'+c_id+'km_id'=+k_id;
    function getPageData($responce){
		$.ajax({
            type: "POST",
            url: $responce,
            datatype: "html",
			data: parameter,
            success: function (data) {

				$("#syana").html(data);
}
        });
	
}


</script>

waiting ur replay