php不能產生類似.pid的標識程序的暫存文件

$filename = "/tmp/test_php.pid";
$fp = fopen($filename, 'w');
//unset($fp);
unlink($filename);
sleep(6);

2，C程序代码

#include
int main()
{
    char *filename = "/tmp/test_c.pid";
    FILE *fp;
    fp = fopen(filename, "w");
    unlink(filename);
    sleep(6);
}