rusty.php

<?php 
$numbers = [ 3,3,2,2,56,4,4 ];

$checked = [];
foreach( $numbers as $number ) {
    $checked[ $number ] = !empty( $checked[ $number ] ) ? $checked[ $number ] + 1 : 1;
}
asort( $checked );

echo current( array_keys( $checked ) );