Description:
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While trying to make my legacy code working with a brand new PHP 7.1 using debian, I discovered that defining a session handler breaks the sessions a,nd throw the following warning: 

“Warning: session_start(): Failed to read session data: user (path: )”

I tried with an empty session_handler, using php 7.0 in order to identify the source of the problem: It worked.

As soon as I upgrade to 7.1 (or 7.2 RC), my sessions stop working.

Test script:
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https://gist.github.com/loranger/f57ce97e1a60d7d106835f07918f85f9

Actual result:
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 Warning: session_start(): Failed to read session data: user (path: ) in index.php on line 49