Recently, I stumbled across another cause: disallowed characters in a POST array. In my case, I had single-quotes as part of the variable name. My fingers mixed up PHP array syntax and HTML form syntax.

<!-- will cause the error -->
<input type="text" name="fieldname['foo']" value=""> 

<!-- won't cause the error. besides, this is the proper syntax. -->
<input type="text" name="fieldname[foo]" value="">

kalodont points out this can also happen when using accented characters, such as ó or ñ as keys in your array.

The line feed (LF) and carriage return (CR) characters (and their hex code equivalents (%0D and %0A) are forbidden in CodeIgniter’s framework. The hard part is tracking down exactly where that extra line break character lives.

In my case, there was an extra line of blank, barren, not-all-that-obvious white space at the very, very end of one of my controller files, just after the closing ?>.

Related:

• PHP header error messages
• Four common P H P errors and what they mean