- var_dump: documentation ( source)
- array_values: documentation ( source)
- array_keys: documentation ( source)
- array_diff: documentation ( source)
- key: documentation ( source)
<?php
$arr = array (
0 => 'nested array',
1 => 'nested array',
2 => 'nested array',
3 => 'nested array',
4 => 'nested array',
5 => 'nested array',
);
$arr[null] = 'jam';
var_dump(key(array_diff(array_keys(array_values($arr)), array_keys($arr))))?:NULL;