linux/cronjob/bash script expert needed for small script

i need to run a php script every 10 to 15 seconds to import data from a text file into a mysql database (the php file already exists). this should be done with a cronjob/bash script (also existing). the cron can run every 5 minutes for example. currently the bash script starts the import as expected (running 30 times with 10 seconds sleep between the imports, making a total execution duration of about 5 minutes). problem is that after five minutes of correct imports the script stops for five minutes and continues again after five minutes. i guess that the script can't be executed because the previous execution is still running? what i need is some solution that can be used with a cronjob and that makes sure that my php import script runs every 10 or 15 seconds in a reliable way. any linux experts out there who can help me with this? please note: you won't have direct access to my server - you need to send all files for me to upload and test.