Auto Index in PHP is a feature of the Apache web server which allows directory browsing. When enabled, a user visiting a directory that does not contain an index file such as index.html or index.php will be presented with a list of files and subdirectories contained in the directory. To enable Auto Index in PHP, you need to edit the Apache configuration file and add a DirectoryIndex directive. This directive will specify which file should be displayed when a user visits a directory. It is also important to make sure that the AllowOverride directive is set to All in order for the DirectoryIndex directive to take effect.

Auto-indexing in PHP refers to the ability of the PHP interpreter to automatically assign a numeric index to each element in an array, starting from 0. This is the default behavior of arrays in PHP, and it can be very useful when working with data that is naturally ordered, such as a list of items or a sequence of numbers.

For example, you can create an array with auto-indexing in PHP like this:

Copy code$fruits = array('apple', 'banana', 'orange');

Here, the elements ‘apple’, ‘banana’, and ‘orange’ will be assigned the index 0, 1, and 2, respectively.

You can access the elements of an auto-indexed array using their numeric index:

Copy codeecho $fruits[0];  // outputs "apple"
echo $fruits[1];  // outputs "banana"
echo $fruits[2];  // outputs "orange"

You can also use the foreach loop to iterate over the elements of an auto-indexed array:

Copy codeforeach ($fruits as $fruit) {
    echo $fruit . "\n";
}

This will output:

Copy codeapple
banana
orange

Note that you can use negative indexing with auto indexed arrays, where -1 is the last element , -2 second last and so on

Copy codeecho $fruits[-1]; // outputs orange

You can also use the array_values() function to re-index an array, so that its keys start at 0 and increment by 1. This can be useful if you’ve used non-numeric keys, or if the keys are not consecutive integers.

Copy code$fruits = array('a'=>'apple', 'b'=>'banana', 'c'=>'orange');
$fruits = array_values($fruits);

The array now looks like

Copy code['apple','banana','orange']

You can use the array_values() function, even when the array is already indexed

Copy codearray_values($fruits); 

This will give the same result, but it does not change the original array, it returns the re-indexed array.