Array Fill as the name suggests is used to fill out the Indices of an Array with values. All you have to do is specify the starting Index, the ending Index and the value that you want to fill out the array with.
The syntax goes something like this-
Array (  => 25  => 25  => 25 )
One could also use this function to Initialize multi dimensional arrays. For example- this is how one would Initialize a 2-Dimensional Array
$a = array_fill(0, 10, array_fill(0, 10, 0));
It is a 10×10 array and all the values are Initialized to 0.