Who had a similar error?

Question

PHP

Hello everyone, who will tell you for what reasons this code

if( isset($array[$key]) )
{
  unset($array[$key]);
}

can cause this error
[error] upstream prematurely closed connection while reading response header from upstream, client: xxxxxxxx, server: xxxxxxxxxx, request: "POST /xxxx/ HTTP/1.1", upstream: "xxxxxxxxxx", host: "xxxxxxxxxx", referrer: "ххххххх" I found an error in error.log because there is a white screen on the site and
it is this line that causes it .
unset($array[$key]);