<input type="text" class="f_taskname" value=""/>

var task_name=$('#filter_body').find('.f_taskname').val();

url:url,
dataType: 'text',
type:'get',
data: { task_name: task_name},
contentType: "application/x-www-form-urlencoded; charset=UTF-8",

$f1=fopen('log.txt', 'w');
$output=$task_name.PHP_EOL;
fwrite($f1, $output);
fclose($f1);

var task_name=$('#filter_body').find('.f_taskname')[0].val();

var taskname = $('.f_taskname').val();