What function is commonly used to open a file in PHP?

The function commonly used to open a file in PHP is fopen(). This function is vital for file handling, as it allows developers to specify the file they want to work with and determine the mode in which the file should be opened (such as read, write, append, etc.).

When fopen() is called, it takes two primary parameters: the filename and the mode. For example, fopen("example.txt", "r") would open the file "example.txt" in read mode. If the file is successfully opened, fopen() returns a file pointer that can be used by other file functions to read from or write to the file.

The other options listed do not represent standard PHP functions for opening files. Functions like file_get_contents() are used to read the entire file into a string, rather than opening a file for writing or reading operations that could be performed later. The options open_file() and file_open() do not exist in the PHP standard library, thus making them incorrect choices.