When do I use PHP_EOL instead of \n and vice-versa ? Ajax/Jquery client problem

The constant PHP_EOL should generally be used for platform-specific output.

• Mostly for file output really.
• Actually the file functions already transform \n ←→ \r\n on Windows systems unless used in fopen(…, "wb") binary mode.

For file input you should prefer \n however. While most network protocols (HTTP) are supposed to use \r\n, that's not guaranteed.

• Therefore it's best to break up on \n and remove any optional \r manually:

  $lines = array_map("rtrim", explode("\n", $content));

  Or use the file(…, FILE_IGNORE_NEW_LINES) function right away, to leave EOL handling to PHP or auto_detect_line_endings.

• A more robust and terser alternative is using preg_split() and a regexp:

  $lines = preg_split("/\R/", $content);

  The \R placeholder detects any combination of \r + \n. So would be safest, and even work for Classic MacOS ≤ 9 text files (rarely seen in practice).

  ^{Obligatory microoptimization note:
  While regex has a cost, it's surprisingly often speedier than manual loops and string postprocessing in PHP.}

And there are a few classic examples where you should avoid PHP_EOL due to its platform-ambiguity:

• Manual generation of network protocol payloads, such as HTTP over fsockopen().
• For mail() and MIME construction (which really, you shouldn't do tediously yourself anyway).
• File output, if you want to consistently write just Unix \n newlines regardless of environment.

So use a literal "\r\n" combination when not writing to files, but preparing data for a specific context that expects network linebreaks.

PHP_EOL should be used when writing output such as log files.

It will produce the line break specific to your platform.

PHP_EOL is a constant holding the line break character(s) used by the server platform. In the case of Windows, it's \r\n. On *nix, it's \n. You apparently have a Windows server.

If you were on a *nix server, that change wouldn't have fixed it, because it would be \n. If you are sending data to the client (i.e. the browser), you should use \r\n to ensure line breaks are recognized.