What's wrong with my bash script to keep last x files and delete the rest?

You could try this one:

ls -r1 $PT_MYSQLBACKUPPATH/ | tail -n +$(($PT_FILESTOKEEP+1)) | xargs rm

ls -r1 will list all files in reverse order, one file per line.

tail -n +$number filters the first $number-1 files of the list out (resp. displays all files beginning from $number till the last one).

xargs will execute rm with all file names from standard input.

First, be sure you are in right folder:

if [ -z $PT_MYSQLBACKUPPATH ]; then
 echo "No PT_MYSQLBACKUPPATH set. Exit"
 exit 1
fi
cd $PT_MYSQLBACKUPPATH
if [ $? != 0 ]; then
 echo "cd to PT_MYSQLBACKUPPATH failed. Exit"
 exit 1
fi

You can remove files older than n, in your case:

find -mtime +14 -delete

Deletes files older than 14 days.

More complicated (definitely not optimal, though) solution for your question:

# Get list of newest files. If newest files are first, use head -n 14 instead of 
# head.
files=(`ls | sort | tail -n 14`)
# Loop over all files in this folder
for i in *; do 
 preserve=0; 
 #Check whether this file is in files array:
 for a in ${files[@]}; do 
  if [ $i == $a ]; then 
   preserve=1; 
  fi; 
 done; 
 # If it wasn't, delete it (or in this case, print filename)
 if [ $preserve == 0 ]; then 
  echo $i; # test first, then change this to "rm $i"
 fi;
done

Here is my usage of inspiration from this post:

#!/bin/bash
# Thu Jun 28 13:22:53 CEST 2012
# ${DESTDIR}/files2keep.sh
# Keep the 3 yungest files
# mra at miracleas.dk , deployed on RHEL 6.
InitValues(){
TODAY=`date +"%Y%m%d"`
NOW=`date +"%H%M"`
DESTDIR=/mnt/dbdmp
LOGFILE=?{0}-${TODAY}-${NOW}.log
}
BackupFileMaintenance(){
KEEPFILES=(`ls -lrt ${DESTDIR}/*mysqldump.sql.gz| tail -n 3| awk '{print $9}'`)
    for i in `ls -lrt ${DESTDIR}/*mysqldump.sql.gz | awk '{print $9}'`; do
    preserve=0 
    #Check whether this file is in files array:
        for a in ${KEEPFILES[@]}; do
                if [ $i == $a ]; then
                preserve=1 
                fi 
        done 
    if [ $preserve == 0 ]; then
    echo $i; # then change this to "rm -f $i" after test
    fi
    done
}
InitValues
BackupFileMaintenance
exit