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<?php $mystring = 'first_group, second_group, third_group'; $results = explode(', ', $mystring); foreach($results as $row){ $expected[] = ['key'=>$row]; } print_r($expected); ?>
based on LsaDS
Output for 7.2.0 - 7.2.33, 7.3.0 - 7.3.23, 7.4.0 - 7.4.13, 8.0.0
Array ( [0] => Array ( [key] => first_group ) [1] => Array ( [key] => second_group ) [2] => Array ( [key] => third_group ) )