Description:
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Starting from v.7.0  after calling json_decode(null), json_last_error() return JSON_ERROR_SYNTAX. For earlier versions it return 0.

https://3v4l.org/VSIXH

Test script:
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<?php

json_decode(null);
var_dump(json_last_error());

Expected result:
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json_last_error() return 0

Actual result:
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json_last_error() return JSON_ERROR_SYNTAX