'unlink', Permission denied error when executing function [exec]

This is the file test1.php:

 <?php    
    set_time_limit(0);
    for($i= 1;$i<5000 ;$i++){

        $comm_sjis = "echo 'test'";
        $result = exec($comm_sjis);

    }
    unset($result);
    echo 'ok';

This is file test2.php:

<?php

set_time_limit(0);

function write_txt($str)
{
    $filepath = 'E:/temp/test.xml';
    if (($fp = @fopen($filepath, "a")) === false) {
        return;
    }

    if (!flock($fp, LOCK_EX)) {
        @fclose($fp);
        return;
    }

    if (fwrite($fp, $str . "\n") === false) {
        @flock($fp, LOCK_UN);
        @fclose($fp);
        return;
    }

    if (!fflush($fp)) {
        @flock($fp, LOCK_UN);
        @fclose($fp);
        return;
    }

    if (!flock($fp, LOCK_UN)) {
        @fclose($fp);
        return;
    }

    if (!fclose($fp)) {
        return;
    }
}

for($i= 1;$i<100 ;$i++){

    write_txt('test');  
    unlink('E:/temp/test.xml');
}
echo 'ok';

If I run file test2.php while test1.php is running, an error will occur:

Warning: unlink(E:/temp/test.xml): Permission denied in C:\xampp\htdocs\test2.php on line 45

When I only run test2.php, without test1.php, this error does not occur. Why does unlink give a Permission denied error when I execute the function?

I'm using XAMPP 3.2 vs php 5.6 with Windows 7.