Pthreads sleep() experiment

pthreads.php

<?php

class MyThread extends Thread
{
    public function run()
    {
        sleep(1);
        echo "ran\n";
    }
}

$start = microtime(true);
for ($i = 0; $i < 5; $i++) {
    echo "before start\n";
    $obj = new MyThread;
    $obj->start();
    echo "after start\n";
}
printf("Run time: ~%ds\n", round(microtime(true) - $start), 1);

# actual output:

/*
before start
after start
before start
ran
after start
before start
ran
after start
before start
ran
after start
before start
ran
after start
Run time: ~4s
ran
*/

# expected:

/*
before start
after start
before start
after start
before start
after start
before start
after start
before start
after start
Run time: ~0s
<after 1 second, instantly these:>
ran
ran
ran
ran
ran
*/