<?php
$old = array( '1', '18', '18', '1', '1', '1', '1', '1', '18', '18');
$new = array(); $count = count( $old) - 1;
for( $i = 0, $j = $i + 1; $i < $count; $i = $j, $j = $i + 1) {
    while( $old[$i] === $old[$j]) $j++;
    $new[] = array_slice( $old, $i, $j - $i);
}
print_r( $new);

Array
(
    [0] => Array
        (
            [0] => 1
        )

    [1] => Array
        (
            [0] => 18
            [1] => 18
        )

    [2] => Array
        (
            [0] => 1
            [1] => 1
            [2] => 1
            [3] => 1
            [4] => 1
        )

    [3] => Array
        (
            [0] => 18
            [1] => 18
        )

)