Confusion with multidimensional arrays and merging - php

Question

I have had success merging two arrays by difference using the following code:
$a=array("2013-08-22"=>"12","2013-08-25"=>"5","2013-08-27"=>"10");
$b=array("2013-08-22"=>"1","2013-08-23"=>"3","2013-08-25"=>"5","2013-08-27"=>"10","2013-08-29"=>"5");
foreach ($b as $key => $value){
if(!array_key_exists($key, $a)){
$a[$key]=0;
}
}
This will return:
Array
(
[2013-08-22] => 0
[2013-08-23] => 0
[2013-08-25] => 5
[2013-08-27] => 10
[2013-08-29] => 0
[2013-12-22] => 12
)
The idea is for a to additionally hold the elements from b that are not present in a.
I am having issues now doing the same thing for the following array format:
$a=array(array("2013-12-22","12"),array("2013-08-25","5"),array("2013-08-27","10"));
$b=array(array("2013-08-22","1"),array("2013-08-23","3"),array("2013-08-25","5"),array("2013-08-27","10"),array("2013-08-29","5"));
I went to try this:
foreach ($b as $key => $value){
if(!array_key_exists($key, $a)){
$a[$key]=array($value[0], 0);
}
}
But the returned result is far from what I need:
Array
(
[0] => Array
(
[0] => 2013-12-22
[1] => 12
)
[1] => Array
(
[0] => 2013-08-25
[1] => 5
)
[2] => Array
(
[0] => 2013-08-27
[1] => 10
)
[3] => Array
(
[0] => 2013-08-27
[1] => 0
)
[4] => Array
(
[0] => 2013-08-29
[1] => 0
)
)
I understand they keys are no longer the dates, but how should I go about checking each array and making sure I don't get double entries?

Answer 1

$a = array(
array("2013-12-22","12"),
array("2013-08-25","5"),
array("2013-08-27","10"));
$b = array(
array("2013-08-22","1"),
array("2013-08-23","3"),
array("2013-08-25","5"),
array("2013-08-27","10"),
array("2013-08-29","5"));
$exists = array();
foreach ($a as $data) {
$exists[$data[0]] = 1;
}
foreach ($b as $data) {
if (array_key_exists($data[0], $exists)) {
continue;
}
$a[] = array($data[0], $data[1]);
}
$a now contains:
array(6) {
[0]=>
array(2) {
[0]=>
string(10) "2013-12-22"
[1]=>
string(2) "12"
}
[1]=>
array(2) {
[0]=>
string(10) "2013-08-25"
[1]=>
string(1) "5"
}
[2]=>
array(2) {
[0]=>
string(10) "2013-08-27"
[1]=>
string(2) "10"
}
[3]=>
array(2) {
[0]=>
string(10) "2013-08-22"
[1]=>
string(1) "1"
}
[4]=>
array(2) {
[0]=>
string(10) "2013-08-23"
[1]=>
string(1) "3"
}
[5]=>
array(2) {
[0]=>
string(10) "2013-08-29"
[1]=>
string(1) "5"
}
}