What will the readfile() function do in PHP?

The readfile() function in PHP is designed to read a file and send its contents directly to the output buffer. When this function is called with a file path as its argument, it reads the entire file and outputs that data as plain text, effectively sending it to the browser or whatever output context is being used. This is particularly useful for serving files to users without having to manually read the file and then echo its contents.

The other options misrepresent the functionality of readfile(). For instance, while it does read a file, it does not do so line by line—this is characteristic of functions like fgets(). Also, readfile() does not open files for reading in the traditional sense, as you might expect with functions like fopen(). Lastly, it does not write data to a file, which is the purpose of functions like fwrite() or file_put_contents(). Thus, the primary purpose of readfile() is effectively to output a file's content, making option B the accurate choice.