thinkphp $_GET string + is filtered into spaces

The value of uid, the string "+", $_GET is printed and filtered into spaces?

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The value of uid, the string "+", $_GET is printed and filtered into spaces?

No filtering occurs, normal URL parsing should be spaces here.
For Base64 strings, because there is +, if you want to pass it through the URL,
Either make sure that the + sign that appears in the URL is %20, that is, replace the + sign with %20,
or use URLSafe’s Base64 encoding, and replace +, /Replace with -,_

URL parsing is to perform urldecode
echo urldecode('+')