When I begin a session, using "session_start()" and then read in a file using file(), readfile(), or require(), the contents of the file get corrupted with double quote characters.

Here's a sample file to read in (name it "test_file.txt"):
<a.b=[]

Here's a PHP webpage that displays corrupted output:
<?php

session_start();
readfile( 'test_file.txt' );

?>

The corrupted output is:
<a.b="[]"

If the call to session_start() is removed from the above script, the output matches the sample file contents.

Also if any of the characters are removed from the sample file, the readfile() works fine, so there's something specific to the text in the sample file that disagrees with session_start().

FYI, the sample file is small section of javascript in an HTML file I am using.  I pared down the javascript/HTML to this small section that was getting corrupted.  I'm trying to echo different HTML files depending on session variables I set.