This has nothing to do with " or ' or \\\. The problem is that PHPStan doesn’t know that path starting with \\ is an absolute path, and tries to expand it to an absolute path, that’s why it shows the message: Path C:\Users\me\wsl$\CentOS7\my\files\here\blah.php does not exist described in the OP. And there might be other assumptions that wouldn’t work with these paths.

This might be a code bug due to the way PHP handles strings in quotes vs. double-quotes, but I have no idea why it only occurs on mapped drives rather than local drives like C:.

Here’s a test PHP script called tryfile.php:

<?php
  $astuff = file_get_contents("T:\x\test.php");
  $bstuff = file_get_contents("Z:\var\www\TDEV\import\shared\post_process_event.php");
  echo ($astuff);
  echo ("\n");
  echo ($bstuff);
?>

It bombs because of the way the backslashes are handled in double-quoted strings:

C:\Temp\x>php tryfile.php
PHP Warning:  file_get_contents(T:\x    est.php): failed to open stream: No such file or directory in C:\Temp\x\tryfile.php on line 2
PHP Warning:  file_get_contents(Z:
ar\www\TDEV\import\shared\post_process_event.php): failed to open stream: No such file or directory in C:\Temp\x\tryfile.php on line 3

I swapped out the double quotes for single quotes:

<?php
  $astuff = file_get_contents('T:\x\test.php');
  $bstuff = file_get_contents('Z:\var\www\TDEV\import\shared\post_process_event.php');
  echo ($astuff);
  echo ("\n");
  echo ($bstuff);
?>

And it works fine: C:\Temp\x>php tryfile.php

<?php

phpinfo();

?>
<?php
require_once('post_process_init.php');
$recs_processed = $db->event->post_process($startDate);
logPostProcessEnd($import_name, $recs_processed);