<?php

$arr = array(
    'firstname' => array('Sara','Jim'),
    'lastname'  => array('Gibson','Jobs')
);

function convertMyArray($arr){
    $new_arr = array();
    $length = count($arr['firstname']);
    for($i = 0; $i < $length; $i++)
    {
        $new_arr[] = array('firstname' => $arr['firstname'][$i], 'lastname' => $arr['lastname'][$i]);
    }
    return $new_arr;
}

print_r(convertMyArray($arr));

?>

Array
(
    [0] => Array
        (
            [firstname] => Sara
            [lastname] => Gibson
        )

    [1] => Array
        (
            [firstname] => Jim
            [lastname] => Jobs
        )

)