<?php
'id' => '1',
'a_date' => '2016-04-30',
),
'id' => '2',
'a_date' => '2016-05-21',
),
'id' => '3',
'b_date' => '2016-05-30',
),
'id' => '4',
'b_date' => '2016-04-02',
),
);
usort( $array, function ( $a, $b ) { $x = ( isset( $a[ 'a_date' ] ) ) ?
'a_date' : 'b_date'; $y = ( isset( $b[ 'a_date' ] ) ) ?
'a_date' : 'b_date'; } );
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