Get earliest Y-m-d date value in a multidimensional array from qualifying rows

I want to get the minimum date from the date column of a multidimensional array, but only from rows with keycode of 0001.

How would I achieve that?

$a = [
  "a" => ['keycode' => '0001','date' => "2015-09-08"],
  "b" => ['keycode' => '0002','date' => "2015-09-05"],
  "c" => ['keycode' => '0003','date' => "2015-09-04"],
  "d" => ['keycode' => '0001','date' => "2015-09-01"],
  "e" => ['keycode' => '0001','date' => "2015-08-01"]
];

Expected result: 2015-08-01

I would first extract all of the values that have a keycode of 0001 and create a new array of just those dates. Then use the php min() function.

PHP Fiddle

$a=array(
  "a" => array('keycode' => '0001','date' =>"2015-09-08"),
  "b" => array('keycode' => '0002','date' =>"2015-09-05"),
  "c" => array('keycode' => '0003','date' =>"2015-09-04"),
  "d" => array('keycode' => '0001','date' =>"2015-09-01"),
  "e" => array('keycode' => '0001','date' =>"2015-08-01")
);

$b = [];

foreach($a as $key => $value){
    if($value['keycode']=='0001'){
     $b[]=$value['date'];
    }
}

print_r($b);

echo min($b);