function sendForm(){
    let v = true;
    let form = document.getElementById('signup');
    for (let i = 0; i < form.children.length; i++){
        form.children[i].removeAttribute('style');
    }
    for (let i = 0; i < form.children.length - 2; i++){
        
        if (form.children[i].value == ''){
            form.children[i].setAttribute('style', 'background-color: pink;');
            v = false;
        }
    }
    if (v){ ajax(); }
}

function ajax(){
    var request = new XMLHttpRequest();
    request.open("POST", "insert.php");
    request.onreadystatechange = function() {
        if(this.readyState === 4 && this.status === 200) {
            alert(this.response);
            document.getElementById('form').setAttribute('style', 'display: none');
        }
    };
    var myForm = document.getElementById("signup");
    var formData = new FormData(myForm);
    request.send(formData);
}<br />
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<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home3/kgdyflwz/public_html/index.php:169) in <b>/home3/kgdyflwz/public_html/index.php</b> on line <b>216</b><br />