Description:
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If called after session_start(), even without the $new_cache_expire argument passed, session_cache_expire() triggers a warning on PHP 7.2 alpha2. The same works fine on PHP 7.1.6 and earlier versions.

Test script:
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<?php

session_start();
session_cache_expire();


Expected result:
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No warning triggered

Actual result:
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Warning: session_cache_expire(): Cannot change cache expire when session is active in test.php on line 4