PHP: Delete files with a specified file extension from a given directory.

You can use the `glob` function in PHP to match file names within a specified directory, and then use the `unlink` function to delete those files.

The specific operating steps are as follows:

1. Use the `glob` function to match file names within a specified directory and store the results in an array; the code is as follows:
   

   $files = glob('/path/to/dir/*.txt');

   Here, `/path/to/dir` refers to the directory to be operated on, and `*.txt` refers to the file name pattern to match – in this case, all files with the `.txt` extension.

2. Iterate over the file array and call the `unlink` function on each file to delete it; the code is as follows:

   foreach ($files as $file) {
       unlink($file);
   }

   The complete code is as follows:

   $files = glob('/path/to/dir/*.txt');
   foreach ($files as $file) {
       unlink($file);
   }

The above code will delete all files with the.txt extension in the specified directory.