Description:
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after fseeking a stream and fwriting less data to it, the previous available data is gone

See https://3v4l.org/2haHa

Test script:
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<?php

$socket = fopen('php://memory', 'a+');
fwrite($socket, 'ab');
fseek($socket, 0);

fwrite($socket, 'a');

var_dump(fgets($socket));

Expected result:
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string(1) "b"

Actual result:
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bool(false)