Description:
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If a session start is not supposed to output any headers, it should not fail in the case if response headers have been already sent.

The script below works on PHP 7.1 and older versions but doesn't work on PHP 7.2.

The same happens when calling session_id($id) before session_start().

Seems similar to #55267.

Test script:
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<?php

ini_set('session.use_cookies', false);
ini_set('session.cache_limiter', false);

echo '.';

session_start();


Expected result:
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No warning triggered.

Actual result:
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Warning: session_start(): Cannot start session when headers already sent in test.php on line 8

Call Stack:
    0.0001     397720   1. {main}() test.php:0
    0.0001     398128   2. session_start() test.php:8