PHP command for issuing UNIX command???

Hi! I wanted to know if there is a PHP code or function that would allow the PHP to issue the UNIX command. What I'm doing here is I'm trying to tell PHP to create a file by issuing hte UNIX command then I can use the PHP to open the file to enter the data. Once I'm done with the file, I can use the UNIX to run the script in the file to communicate with the modem by executing the file. When I'm done then I can open an another file to grab the data that had be received by the modem and do the PHP things to do something with hte data. When done, then I can use the PHP to tell UNIX to delete the file and Voila! Thanks, Scott