$1 represents the value of the brackets located here:
/\[poll(\d+)\]/i. $0 would be the entire matched string, which is typically a useless chunk for something like a preg_replace.
I ran that test above on codepad (I was surprised it worked since they have issues with pattern compiling). The results are a copy/paste of the source it generated. It may be wiser to use backreference with \1 instead of $1.
header('HTTP/1.1 420 Enhance Your Calm');