PHP cannot generate a temporary file similar to .pid that identifies the process

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

2, C program code

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