<?php
 
$data1 = [
    ["1-6-13", 10],
    ["30-6-13", 13],
    ["20-9-13", 28]
];
 
$data2 = [
    ["18-2-13", 7],
    ["30-6-13", 9]
];
 
$dates = array_merge(array_map('reset', $data1), array_map('reset', $data2));
 
$result = array_fill_keys($dates, array(null, null, null));
 
foreach($data1 as $pair) {
    list($date, $num) = $pair;
    $result[$date][0] = $date;
    $result[$date][1] = $num;
}
 
foreach($data2 as $pair) {
    list($date, $num) = $pair;
    $result[$date][0] = $date;
    $result[$date][2] = $num;
}
 
$result = array_values($result);
 
print_r($result);

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