Count all checked checkbox and display the first value

I need help on my php program.

I have 2 forms inside my testing.php

• Main form named "mainform"
• the other Form is with checkbox named "modalForm"

My program works like this. If you click the disabled textbox, a modal will pop-up containing the form with checkbox. If only 1 of the checkbox were check I'm gonna display the value of it on my disabled textbox otherwise if more than 2 were check I will display the value "Multiple" on the disabled textbox.

Here is my look a like code

testing.php

<form method="post" name="mainform" action="">
        <label>TestLabel</label>
        <a href="#modal"> <!-- when the input textbox was clicked, modal will pop up -->
            <input disabled type="text" name="test" placeholder="test" value="">
        </a>
    </form>

    <form method="post" name="modalForm" id="modalForm" action="testing.php">
        <div class="modalwrapper" id="modal">   <!-- modal -->
                <div class="modalcontainer">    
                    <div class="modalcol1">
                        <label>Test 1</label>
                        <input type="checkbox" name="test_modal[]" value="mark">
                        <div class="clear"></div>
                        <label>Test 2</label>
                        <input type="checkbox" name="test_modal[]" value="joe">
                        <div class="clear"></div>
                        <label>Test 3</label>
                        <input type="checkbox" name="test_modal[]" value="kevin">
                        <div class="clear"></div>
                        <label>Test 4</label>
                        <input type="checkbox" name="test_modal[]" value="michael">
                        <div class="clear"></div>
                        <label>Test 5</label>
                        <input type="checkbox" name="test_modal[]" value="jordan">
                        <div class="clear"></div>

                        <div class="savebutton">
                            <input class="btn1" type="submit" value="Submit"/>
                        </div>
                    </div>
                </div>
            <div class="overlay"></div>
        </div>      
    </form>

styles.css

/* modal layout */
    .modalwrapper {
        top: 0;
        left: 0;
        opacity: 0;
        position: absolute;
        visibility: hidden;
        box-shadow: 0 3px 7px rgba(0,0,0,.25);
        box-sizing: border-box;
        transition: all .4s ease-in-out;
        -webkit-transition: all .4s ease-in-out;
        -moz-transition: all .4s ease-in-out;
    }

    .modalwrapper:target {
        opacity: 1;
        visibility: visible
    }

    .overlay {
        background-color: #000;
        background: rgba(0,0,0,.8);
        height: 100%;
        left: 0;
        position: fixed;
        top: 0;
        width: 100%;
        z-index: 1;
    }

    .modalcontainer {
        display: table;
        background-color: #777;
        position: relative;
        z-index: 100;
        color: #fff;
        padding: 5px;
    }

    .modalcol1 { display: table-cell; }

    .clear { clear: both; }

    .modalwrapper input[type=checkbox] {
        float: right;
        margin-right: 20px;
    }

    .savebutton input[type=submit] {
        float: right;
        background-color: maroon;
        color: #fff;
        border: none;
        padding: 5px 10px;
        margin-top: 5px;
        margin-right: 20px;
    }
    /* modal layout ends here */

Hope you guys can help me. I wanna know how can I pass the value of the checkboxes on my mainform . Thank you so much in advance.