3v4l.org

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<?php $array = [ '2018-03-12' => [ 'United States' => 4, 'Australia' => 15, 'United Kingdom' => 0, 'New Zealand' => 0, ], '2018-03-13' => [ 'United States' => 0, 'Australia' => 8, 'United Kingdom' => 2, 'New Zealand' => 0, ] ]; var_export( array_map( fn($k) => array_merge([$k], array_values($array[$k])), array_keys($array) ) );
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.33, 8.2.0 - 8.2.29, 8.3.0 - 8.3.27, 8.4.1 - 8.4.14
array ( 0 => array ( 0 => '2018-03-12', 1 => 4, 2 => 15, 3 => 0, 4 => 0, ), 1 => array ( 0 => '2018-03-13', 1 => 0, 2 => 8, 3 => 2, 4 => 0, ), )
preferences:
140.84 ms | 407 KiB | 5 Q