Why 2 http request to the PHP are executed in sequential order, not simultaneously?

Wrote in index.php this:
sleep(5);
echo date('-r');

Request this file in 2 browser tabs at the same time. In the second tab, the answer is always loaded only after 5 seconds after loading the first time. Thought it was me so configured, zaregal free hosting, and there, too, so. So requests for PHP scripts are executed in sequential, not in parallel. Why???
April 4th 20 at 13:03
3 answers
April 4th 20 at 13:05
Solution
Indeed, unambiguous sequence.
But if you make the URL unique (via index.php?_=random()) - sequence anymore.
Already noticed it. And by the way happens in Chrome, in Firefox, tabs are always loaded in parallel. - Kory commented on April 4th 20 at 13:08
I have an old Firefox v47, and in it one at a time. - aubree_Lakin commented on April 4th 20 at 13:11
April 4th 20 at 13:07
if php starts a session it blocks the script execution until the completion of the past.
Session does not start:
session_id() gives an empty string and session.auto_start = Off - Kory commented on April 4th 20 at 13:10
April 4th 20 at 13:09
Because the session must be closed before going to sleep.
Session does not start:
session_id() gives an empty string and session.auto_start = Off - Kory commented on April 4th 20 at 13:12

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