test_array_group.php

<?php
$array = [
  [ 'id' => 1, 'value' => 'foo' ],
  [ 'id' => 1, 'value' => 'bar' ],
  [ 'id' => 2, 'value' => 'baz' ],
];

$groups = array_group( $array, function( $a, $b ) {
  return $a['id'] == $b['id'];
} );

var_dump( $groups );
/*
array(2) {
  [0]=>
  array(2) {
    [0]=>
    array(2) {
      ["id"]=>
      int(1)
      ["value"]=>
      string(3) "foo"
    }
    [1]=>
    array(2) {
      ["id"]=>
      int(1)
      ["value"]=>
      string(3) "bar"
    }
  }
  [1]=>
  array(1) {
    [0]=>
    array(2) {
      ["id"]=>
      int(2)
      ["value"]=>
      string(3) "baz"
    }
  }
}
*/

test_array_incr_subseqs.php

<?php
function less_than_or_equal( $a, $b ) {
	return $a <= $b;
}

$arr1 = array(1,2,2,3,1,2,0,4,5,2);

var_dump( array_group($arr1, 'less_than_or_equal') );
/*
array(4) {
  [0]=>
  array(4) {
    [0]=>
    int(1)
    [1]=>
    int(2)
    [2]=>
    int(2)
    [3]=>
    int(3)
  }
  [1]=>
  array(2) {
    [0]=>
    int(1)
    [1]=>
    int(2)
  }
  [2]=>
  array(3) {
    [0]=>
    int(0)
    [1]=>
    int(4)
    [2]=>
    int(5)
  }
  [3]=>
  array(1) {
    [0]=>
    int(2)
  }
}
*/

test_datetime_group.php

<?php
function event( $start, $end ) {
	return (object)array( 'start' => strtotime( $start ), 'end' => strtotime( $end ) );
}

function overlaps( $ev1, $ev2 ) {
	$start_time1 = $ev1->start;
	$start_time2 = $ev2->start;
	$end_time1   = $ev1->end;
	$end_time2   = $ev2->end;

	return ($start_time1 <= $end_time2 && $start_time2 <= $end_time1);
}

$events = [
	event( '2023-05-01 00:00:00', '2023-05-01 01:00:00' ),
	event( '2023-05-01 00:30:00', '2023-05-01 01:15:00' ),
	event( '2023-05-01 02:00:00', '2023-05-01 03:00:00' ),
];

$grouped = array_group( $events, 'overlaps' );

var_dump( $grouped ); // produces [ [ $events[0], $events[1] ], [ $events[2] ] ]

test_manual_group.php

<?php
$array = [
  [ 'id' => 1, 'value' => 'foo' ],
  [ 'id' => 1, 'value' => 'bar' ],
  [ 'id' => 2, 'value' => 'baz' ],
];

$groups = [];
foreach ( $array as $element ) {
   $groups[ $element['id'] ][] = $element;
}

var_dump( $groups );
/*
array(2) {
  [1]=>
  array(2) {
    [0]=>
    array(2) {
      ["id"]=>
      int(1)
      ["value"]=>
      string(3) "foo"
    }
    [1]=>
    array(2) {
      ["id"]=>
      int(1)
      ["value"]=>
      string(3) "bar"
    }
  }
  [2]=>
  array(1) {
    [0]=>
    array(2) {
      ["id"]=>
      int(2)
      ["value"]=>
      string(3) "baz"
    }
  }
}
*/