Saving an array as a file

Okay, here's my last dumb newbie question for today - maybe. I see how to use file() to open a text file as an array of strings with each line an array element. That's great. So I make my changes with whatever code to one or more array elements. Now, um, what do I do to save the changes back to the file? Example (not exactly what I'm really doing, but way simplified for clarity): //open file as an array $arrTextFile = file("test.txt"); //modify array $arrText[0] = ereg_replace("this","that",$arrText[0]); $arrText[1] = ereg_replace("this other","that other",$arrText[1]); //save modified array back to test.txt file ??so what goes here?? Maybe there's a better way to do this, I don't know. But I need to find a way to 1)find a specific line in a file (by matching a string value), modify a different portion of that line only - replacing part of the string with something else, and then saving the file. TIA -Jeff