difference between , =!, ==! in php

difference between , =!, ==! in php

Hi I am doing a conditional
if ($row['ConsignadaCaja'] === 'si' && $row['Estado'] ==! 'I') {
$pago = 0;
}
It does not work as I want
So I try using
if ($row['ConsignadaCaja'] === 'si' && $row['Estado'] =! 'I') {
$pago = 0;
}
But it does not work either
Finally I try this:
if ($row['ConsignadaCaja'] === 'si' && $row['Estado'] <> 'I') {
$pago = 0;
}
It is works but do not why?