http-php the URL argument (the full period becomes an underscore), for the sake of explanation.

inside the . get parameter (full period in English) will become _ (underline in English).
I carefully analyzed the problem and found that it was true.

Online test Address:
http://moonlordapi.sinaapp.com/test.php?.=1
(Don't believe yourself.) )

So my question now is ...
1. Why is this? Is there any official explanation or description for this question?
2. Are there any other special symbols that can show such wonderful phenomena?

———————————————————— Gorgeous split-line ————————————————————

UPDATE:
The answer has been adopted.
But look http://php.net/manual/en/language.variables.external.php , that note was 7 years ago ...
I measured it myself (PHP version here is 5.4+):

The parameters of the space and the left bracket are ignored (and the description is different).
CHR-Chr (159) is not a common text character, it is not tested ...

Reply content:

inside the . get parameter (full period in English) will become _ (underline in English).
I carefully analyzed the problem and found that it was true.

Online test Address:
http://moonlordapi.sinaapp.com/test.php?.=1
(Don't believe yourself.) )

So my question now is ...
1. Why is this? Is there any official explanation or description for this question?
2. Are there any other special symbols that can show such wonderful phenomena?

———————————————————— Gorgeous split-line ————————————————————

UPDATE:
The answer has been adopted.
But look http://php.net/manual/en/language.variables.external.php , that note was 7 years ago ...
I measured it myself (PHP version here is 5.4+):

The parameters of the space and the left bracket are ignored (and the description is different).
CHR-Chr (159) is not a common text character, it is not tested ...

In PHP, you cannot use a period point (.) in a variable name. For example $a.b , a variable name that is not valid. As a result, PHP automatically replaces the dots with underscores.

In addition to the point, some other characters will be automatically replaced with underscores if they appear in the Get parameter name. The word list characters will be replaced automatically:

chr(32) ( ) (空格)chr(46) (.) (点)chr(91) ([) (中括号)chr(128) - chr(159) (多个字符)

Reference: http://php.net/manual/en/language.variables.external.php

