If a checkbox is checked check another box too

I want to try make so it that if checkbox two is checked then checkbox three is check too, and if three is checked then check box one two and three are checked. I'm fairly new to php. Any ideas how i'd do this with html and php? Thanks for looking.

Here is the code I have so far.

    <span class="star-rating">
   <!--RADIO 1-->
    <input type='checkbox' class="radio_item" value="1" name="item" id="radio1">
        <label class="label_item" for="radio1"> <img src="label.png" style="width:30px;height:28px"> </label>

    <!--RADIO 2-->
    <input type='checkbox' class="radio_item" value="2" name="item2" id="radio2">
    <label class="label_item" for="radio2"> <img src="label.png" style="width:30px;height:28px"> </label>

      <!--RADIO 3-->
    <input type='checkbox' class="radio_item" value="3" name="item3" id="radio3">
    <label class="label_item" for="radio3"> <img src="label.png" style="width:30px;height:28px"> </label>


      <!--RADIO 4-->
    <input type='checkbox' class="radio_item" value="4" name="item4" id="radio4">
    <label class="label_item" for="radio4"> <img src="label.png" style="width:30px;height:28px"> </label>

      <!--RADIO 5-->
    <input type='checkbox' class="radio_item" value="5" name="item5" id="radio5">
    <label class="label_item" for="radio5"> <img src="label.png" style="width:30px;height:28px"> </label>
</span>

<?php

if (isset($_POST['item5'])) {
    isset($_POST['item4'];
}