array_unique() not making the array unique - php

Question

I am generating an array of numbers based on an equation then rounding to the nearest 100.
After that I want to get rid of duplicates, array_unique seemed like the natural choice for this situation, but is not working as intended.
I created a small sample to demonstrate this.
The PHP code is as follows:
var_dump($amounts);
array_unique($amounts);
var_dump($amounts);
The result of which is:
array(6) {
[0]=>
float(200)
[1]=>
float(300)
[2]=>
float(300)
[3]=>
float(400)
[4]=>
float(500)
[5]=>
float(500)
}
array(6) {
[0]=>
float(200)
[1]=>
float(300)
[2]=>
float(300)
[3]=>
float(400)
[4]=>
float(500)
[5]=>
float(500)
}
Can someone shed some light on what is happening here please?

Answer 1

array_unique does not modify the array by reference. You'll need to catch the returned value:
$amounts = array_unique($amounts);
Note: the keys of the returned array may no longer be contiguous. If you want to make them contiguous again then you should use array_values.
Example:
$amounts = array(100, 200, 200, 200, 300, 400);
var_dump($amounts);
array(6) {
[0]=>
int(100)
[1]=>
int(200)
[2]=>
int(200)
[3]=>
int(200)
[4]=>
int(300)
[5]=>
int(400)
}
// Make the array unique
$amounts = array_unique($amounts);
var_dump($amounts);
array(4) {
[0]=>
int(100)
[1]=>
int(200)
[4]=>
int(300) // Notice the gap, indexes 2 and 3 don't exist.
[5]=>
int(400)
}
// Make the keys contiguous
$amounts = array_values($amounts);
var_dump($amounts);
array(4) {
[0]=>
int(100)
[1]=>
int(200)
[2]=>
int(300)
[3]=>
int(400)
}