How to Initiate the Submit button function

new guy here. i'm currently developing a form for web scraping activities. may i ask, how do i initiate the submit button function using isset? tried a few methods but to no avail. so sorry if this question seems basic, but i really want to learn the fundamentals in order to proceed with bigger form function in the future. here's my code:

<form id="startWeb" name="startWeb" method="post" action="webcrawlBM_process.php">
<input type="button" id="startCrawl" action="submit" value="Mula Mengikis" class="gradient-background" onClick="submitForm();" style="width: 230px" />

when user click on submit, a javascript function will initiate.

function submitForm(action)
{
    document.getElementById('startWeb').action = action;
    document.getElementById('startWeb').submit();
    alert('Data anda telah disimpan!  Anda akan dihalakan kembali ke halaman utama.');
    window.location = 'index.php';
}

here's my isset function that is problematic. if (isset($_POST['submit'])) {

$webcrawl = new WebCrawlerBM($_POST['target_url']);

if ($webcrawl->WebCrawlerBM($_POST['target_url'])) {
if ($webcrawl->submitForm()) {
    $webcrawl->set_webpage($url);
    $webcrawl->set_makefile($text); 
    $webcrawl->set_writefile($filename);
    $webcrawl->set_striphtmltags($text);

    echo "Your action had been completed successfully.  <br>";

} 

can anyone give me a guidance on this? been stuck on this error for a while. the situation now is that when i clicked, nothing happened. please, kind guidance is appreciated.