array_unique() takes input
array and returns a new array
without duplicate values.
Note that keys are preserved. array_unique() sorts
the values treated as string at first, then will keep the first key
encountered for every value, and ignore all following keys. It does not
mean that the key of the first related value from the unsorted
array will be kept.
Note: Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same.
The first element will be used.