Greetings!

I have here code that sends values via ajax. That values includes an array.

Code:
<script type = 'text/javascript'>
function insert() {
if(window.XMLHttpRequest){
 xmlhttp = new XMLHttpRequest();
} else {
xmlhtttp = new ActiveXObject('Microsoft.XMLHttp');
}


xmlhttp.onreadystatechange = function() {
		if (xmlhttp.readyState ==4 && xmlhttp.status == 200) {
		document.getElementById('message').innerHTML = xmlhttp.responseText;
		}
}

parameters = 'keyword='+document.getElementById('keyword').value + '&accnum='+document.getElementsByName('accnum[]').value;
/*"parameter1=" + parameter1.value + "&parameter2=" + parameter2.value*/

xmlhttp.open('POST', 'Borrow_2.php', true);
xmlhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
xmlhttp.send(parameters);
		
		}

</script>
<script type = 'text/javascript'>
function addRow()
 {
     tabBody=document.getElementById("inputtable");
	row=document.createElement("TR");
    cell1 = document.createElement("TD");
    textnode1=document.createElement("input");
    textnode1.name="accnum[]";
	textnode1.type="text";
    cell1.appendChild(textnode1);
    row.appendChild(cell1);
    tabBody.appendChild(row);

}
</script>
 </HEAD>
 <BODY>
<form id="test" onsubmit="return false;" autocomplete="off">
 <div class="main">

  
   <table>
  <tbody id="inputtable">
     	 <div id="holder"> 
		 <p>Student ID: <input type="text" id="keyword" tabindex="0" name = 'keyword'></p>
		  <div id="ajax_response"></div>
		 <tr><td>SAccession No: <input type="text" id="accnum" tabindex="0" name = 'accnum[]' >
		 <input type="button" value="Add new" onclick="addRow();" /></td></tr>		 
				
				<br/>
				<br/>
				<br/>
		        
				 
		
		
		  <input type="submit" value="submit" onClick="insert();"/>
My problem is I can't get the value of my accnum[].
I think the problem is this line, and I don't know how to fix it.
Code:
parameters = 'keyword='+document.getElementById('keyword').value + '&accnum='+document.getElementsByName('accnum[]').value;
When I used var_dump(); it says that 'undefined strings'. What's that mean? thank you.