Don't run cron job if already running
Please see Process Management. You will probably want to use a lock file. You can use flock for example.
Why can't you let your jobs complete?
Your questions says "I either didn't understand the answer or it didn't apply to me", but you don't say in what way. Please be more specific.
Use a bash script as your cron job
cron.sh
#!/bin/bash
$lockfile=/tmp/lock.file
if [ ! -e $lockfile ]; then
touch $lockfile
php /path/to/php/script.php
rm $lockfile
else
echo "script already running"
fi