Sort array by date

sort_array.php

<?php
/**
* In this case, the value to sort is a date in string, i.e. "01/01/2018" or "2017-12-31"
*/

/**
 * Example of array
 */
$array = array( 
  array( "01/01/1968", "Do Androids Dream of Electric Sheep?", "Philip K. Dick", "Philosophical novel" ),
  array( "16/08/2011", "Ready Player One", "Ernest Cline", "Dystopian" ),
  array( "01/01/1951", "Foundation", "Isaac Asimov", "Science Fiction" ),
  );

/**
 * Comparison function to sort the array by date
 */
function sort_comparison( $a, $b ) {
  return strtotime( $a[0] ) - strtotime( $b[0] );
}

usort( $array, 'sort_comparison' );
/**
 * Result
 *
 * array(3) {
 *   [0]=>
 *   array(4) {
 *     [0]=>
 *     string(10) "01/01/1951"
 *     [1]=>
 *     string(10) "Foundation"
 *     [2]=>
 *     string(12) "Isaac Asimov"
 *     [3]=>
 *     string(15) "Science Fiction"
 *   }
 *   [1]=>
 *   array(4) {
 *     [0]=>
 *     string(10) "01/01/1968"
 *     [1]=>
 *     string(36) "Do Androids Dream of Electric Sheep?"
 *     [2]=>
 *     string(14) "Philip K. Dick"
 *     [3]=>
 *     string(19) "Philosophical novel"
 *   }
 *   [2]=>
 *   array(4) {
 *     [0]=>
 *     string(10) "16/08/2011"
 *     [1]=>
 *     string(16) "Ready Player One"
 *     [2]=>
 *     string(12) "Ernest Cline"
 *     [3]=>
 *     string(9) "Dystopian"
 *   }
 * }
 */