$sections = array(
    0 => 'section_one',
    1 => 'section_two',
    2 => 'section_three',
    3 => 'section_four',
    4 => 'section_five',
    5 => 'section_six',
    6 => 'section_seven'
);

array(7) { [0]=> string(11) "section_one" [1]=> string(11) "section_two" [2]=> string(13) "section_three" [3]=> string(12) "section_four" [4]=> string(12) "section_five" [5]=> string(11) "section_six" [6]=> string(13) "section_seven" }

$arr[ $newkey ] = $arr[ $oldkey ];
unset( $arr[ $oldkey ] );

$sections_a = array(
    0 => 'section_one',
    1 => 'section_six',
    2 => 'section_two',
    3 => 'section_three',
    4 => 'section_four',
    5 => 'section_five',
    6 => 'section_six',
    7 => 'section_seven'
);