The passthru() function is similar to the exec() function in that it executes a command . This function should be used in place of exec() or system() when the output from the Unix command is binary data which needs to be passed directly back to the browser. A common use for this is to execute something like the pbmplus utilities that can output an image stream directly. By setting the Content-type to image/gif and then calling a pbmplus program to output a gif, you can create PHP scripts that output images directly.