Description:
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Documentation for session_write_close() states that it returns boolean value but according to this https://github.com/php/php-src/commit/7f196e321 this statement is true only for version >= 7.2. For earlier versions this function returns nothing. I assume reference to return value should specify that earlier versions don't return anything and that boolean return value is available for versions 7.2 and higher.

Test script:
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// for php versions before 7.2
session_start();
$result = session_write_close();
var_dump($result); 

Expected result:
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<script_name>:<line>:boolean true


Actual result:
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<script_name>:<line>:null