AJAX和PHP崩溃服务器

I have a simple page with a DB table that is updated every second using a AJAX POST request to a PHP page. After 40/50 minutes the server crashes and it is out of service for 1 hour. Is it a known issue?

function runAjax(){
    var filename = "readDB.php";
    $.ajax({
        type: "POST",   
        url: filename,   
        data: abc_params,
        success: function(msg){   
             processResponse(msg);
        }
    });   
}
function processResponse(txt){  
    dosomething(txt);
    setTimeout("runAjax()",1000);
}
setTimeout("runAjax()",1000);

readDB.php simply returns a string with all fields of 1 row of 1 table of the DB

UPDATE: I tried also with a "readDB.php" page like this:

<?echo "OK";?>

After 2 hours the site php pages give error 500 but only after 1 minute the site works correctly again.

Here may be problem with mysql connection limit, So please follow like :

<?php
$link = mysql_connect('localhost', 'mysql_user', 'mysql_password');
if (!$link) {
    die('Could not connect: ' . mysql_error());
}
echo 'Connected successfully';
mysql_close($link);
?>

using "mysql_close" it will terminate connection