PHP provides the array_rand( $arr, $num )  function which returns an integer or array containing the keys of the array.

To obtain an array of values the following one-liner can be used:

array_intersect_key( $arr, array_flip( array_rand( $arr, $num ) ) );

This picks $num random keys from $arr (array_rand), flips keys with values (array_flip), intersects the picked keys with those of $arr (array_intersect_keys) and returns the corresponding elements of $arr.

Getting random values from an array.

This works for $num > 1 as array_rand returns an integer otherwise. When a single random value is required, the expression becomes simpler:

$arr[ array_rand( $arr ) ];