Generic.PHP.Syntax false negative when file content passed via STDIN #915

Describe the bug

The Generic.PHP.Syntax sniff is unable to check for syntax errors when the file content is passed via STDIN, resulting in a false negative.

To reproduce

Steps to reproduce the behavior:

1. Run echo '<?php $array = [1, 2, 3; // Missing closing bracket.' | phpcs --standard=Generic --sniffs=Generic.PHP.Syntax -
2. No errors are displayed.

Expected behavior

I believe the command above should display an error as there is a syntax error in the provided PHP code (missing closing bracket).

Additional information

This is happening because the sniff does not consider that $phpcsFile->getFilename() might return STDIN instead of a file name:

When STDIN is passed to the php -l command, PHP returns an error saying that the file does not exist, and the sniff ends its execution without generating an error.

I think the sniff could be modified to run a different command when the file content is passed via STDIN (I'm not sure if this would work on Windows):

echo '<?php $array = [1, 2, 3; // Missing closing bracket.' | /usr/bin/php7.4 -l -d display_errors=1 -d error_prepend_string='' 2>&1

Versions (please complete the following information)

Please confirm

• I have searched the issue list and am not opening a duplicate issue.
• I have read the Contribution Guidelines and this is not a support question.
• I confirm that this bug is a bug in PHP_CodeSniffer and not in one of the external standards.
• I have verified the issue still exists in the master branch of PHP_CodeSniffer.