PHP inserting carriage returns into POST values?

Hey everyone. I ran into a really weird issue that I was hoping I could find some clarification on. In short, I have javascript functions that operate on hidden text values. Those values may be posted, in which case PHP then prints them back to the page via what comes in on $_POST. The weird thing is, I was no longer able to match substrings inside those hidden text values after posting via Javascript. I banged my head over this for a couple hours, until I realized that the string's length was being increased by one after posting (I found this out in javascript). Upon further investigation, I found that all instances of "substring\n" were being replaced by "substring(carriage return)\n" after post. For now, I'm simply doing str_replace(chr(13), "", $_POST['value']) before re-inserting it into the HTML, but I was wondering why PHP is inserting those extra characters. Thanks! James