Online PHP editor | output for XO97G

@ 2020-02-27T22:09:42Z

<?php
$array = [
    ['product_id' => 52, 'date' => '2017-07-28'],
    ['product_id' => 53, 'date' => '2017-07-30'],
    ['product_id' => 81, 'date' => '2017-07-26'],
    ['product_id' => 123, 'date' => '2017-07-30'],
    ['product_id' => 59, 'date' => '2017-07-26'],
    ['product_id' => 124, 'date' => '2017-07-30']
];

foreach ($array as $subarray) {
    $date = $subarray['date'];                         // not strictly necessary, but may aid code readability
    if (!isset($result[$date])) {
        $result[$date] = $subarray;                    // first encounter means no index
    } elseif (isset($result[$date]['product_id'])) {   // if associative, make indexed with associative subarrays
        $result[$date] = [$result[$date], $subarray];  // second encounter means structural change to indexed subarrays
    } else {
        $result[$date][] = $subarray;                  // beyond second encounter means push subarray into indexed structure
    }
}
krsort($result);                                       // it appears that you want DESC order
var_export($result);

Output for 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.19, 8.3.0 - 8.3.4, 8.3.6 - 8.3.7: array ( '2017-07-30' => array ( 0 => array ( 'product_id' => 53, 'date' => '2017-07-30', ), 1 => array ( 'product_id' => 123, 'date' => '2017-07-30', ), 2 => array ( 'product_id' => 124, 'date' => '2017-07-30', ), ), '2017-07-28' => array ( 'product_id' => 52, 'date' => '2017-07-28', ), '2017-07-26' => array ( 0 => array ( 'product_id' => 81, 'date' => '2017-07-26', ), 1 => array ( 'product_id' => 59, 'date' => '2017-07-26', ), ), )
Output for 8.3.5: Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0 array ( '2017-07-30' => array ( 0 => array ( 'product_id' => 53, 'date' => '2017-07-30', ), 1 => array ( 'product_id' => 123, 'date' => '2017-07-30', ), 2 => array ( 'product_id' => 124, 'date' => '2017-07-30', ), ), '2017-07-28' => array ( 'product_id' => 52, 'date' => '2017-07-28', ), '2017-07-26' => array ( 0 => array ( 'product_id' => 81, 'date' => '2017-07-26', ), 1 => array ( 'product_id' => 59, 'date' => '2017-07-26', ), ), )

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