Find a matching or closest value in an array

A particular lazy approach is having PHP sort the array by the distance to the searched number:

$num = 3;    
$array = array(0, 5, 10, 11, 12, 20);
$smallest = [];

foreach ($array as $i) {
    $smallest[$i] = abs($i - $num);
}
asort($smallest);
print key($smallest);

Pass in the number you're searching for as the first parameter and the array of numbers to the second:

function getClosest($search, $arr) {
   $closest = null;
   foreach ($arr as $item) {
      if ($closest === null || abs($search - $closest) > abs($item - $search)) {
         $closest = $item;
      }
   }
   return $closest;
}