3v4l.org

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<?php $a = array( '123' => array(1) ); $b = array( '123' => array(2, 3) ); foreach ($b as $k => $v) { $a[$k] = array_merge($a[$k] ?? [], $v); } var_export($a);
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.34, 8.2.0 - 8.2.30, 8.3.0 - 8.3.30, 8.4.1 - 8.4.18, 8.5.0 - 8.5.3
array ( 123 => array ( 0 => 1, 1 => 2, 2 => 3, ), )
preferences:
76.16 ms | 1244 KiB | 4 Q