PHP usort collections with varying attributes - php

Question

I have two identical arrays:
$array1 = array(
array(
'start' => 1,
'value' => 10
),
array(
'start' => 8,
'value' => 4
),
array(
'start' => 4,
'value' => 8
),
array(
'value' => 5,
)
);
$array2 = array(
array(
'start' => 1,
'value' => 10
),
array(
'start' => 8,
'value' => 4
),
array(
'start' => 4,
'value' => 8
),
array(
'value' => 5,
)
);
I need to merge and sort them in ascending order:
$array1 = array_merge($array1, $array2);
usort($array1, function($value1, $value2) {
if (!array_key_exists('start', $value1) || !array_key_exists('start', $value2)) {
return 0;
}
return $value1['start'] - $value2['start'];
});
However, this doesn't work, and the ordering is incorrect:
Array
(
[0] => Array
(
[start] => 4
[start] => 8
)
[1] => Array
(
[start] => 8
[value] => 4
)
[2] => Array
(
[amount] => 5
)
[3] => Array
(
[start] => 1
[value] => 10
)
[4] => Array
(
[value] => 5
)
[5] => Array
(
[start] => 1
[value] => 10
)
[6] => Array
(
[start] => 4
[value] => 8
)
[7] => Array
(
[start] => 8
[value] => 4
)
)
It should ideally place all items without a start attribute to the start of the array and all items with a start attribute following those in ascending order.
If I remove the arrays that contain no start attribute, it seems to work perfectly. But I need to keep those items without a start attribute. Is there anything I can do with my sort to go around this issue without going through the merged array, removing those that have no start and replacing them after the sort? Ideally I'd love to understand what is happening here to make it fail.

Answer 1

To place the elements without an start key at the front, you have to handle them diffrently in your compare function.
You could try something like this
usort($array1, function($value1, $value2) {
if (!isset($value1['start']) && !isset($value2['start'])) {
return 0;
} else if (!isset($value1['start'])) {
return -1;
} else if (!isset($value2['start'])) {
return 1;
}
return $value1['start'] - $value2['start'];
});