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@ 2020-08-14T05:19:22Z

<?php

$subjects = array(
       array(
           'class' => 'one',
           'sub' => 'music',
       ),
           array(
           'class' => 'one',
           'sub' => array( 'social', 'health', 'science' ),
       ),
          array(
           'class' => 'two',
           'sub' => 'music',
       ),
           
           array(
           'class' => 'one',
           'sub' => 'math',
       )    
    );
    
   $class_sub = array();
    $result = array();
    
    foreach( $subjects as $sub ) {
        if ( ! isset( $class_sub[ $sub['class'] ] ) ) {
            $class_sub[ $sub['class'] ] = $sub['sub'];
        } else {
            if ( is_array( $class_sub[ $sub['class'] ] ) ) {
                $new = array_merge( $class_sub[ $sub['class'] ], is_array($sub['sub']) ?  $sub['sub'] : array($sub['sub']));
            } else {
                $new = array_merge( array($class_sub[ $sub['class'] ]),  is_array($sub['sub']) ?  $sub['sub'] : array($sub['sub']));
            }
            
            $class_sub[ $sub['class'] ] = $new;
        }
    }
    
    foreach( $class_sub as $class => $sub ) {
        $result[] = array(
            'class' => $class,
            'sub' => $sub
        );
    }
    
echo "<pre>"; print_r( $result ); echo "</pre>";

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 - 8.2.18, 8.3.0 - 8.3.4, 8.3.6: <pre>Array ( [0] => Array ( [class] => one [sub] => Array ( [0] => music [1] => social [2] => health [3] => science [4] => math ) ) [1] => Array ( [class] => two [sub] => music ) ) </pre>
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 <pre>Array ( [0] => Array ( [class] => one [sub] => Array ( [0] => music [1] => social [2] => health [3] => science [4] => math ) ) [1] => Array ( [class] => two [sub] => music ) ) </pre>

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