Compare two 2D arrays & get intersection and differences

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Convert arrays to a format, where array index is the sight_id:

$b1 =array();
foreach($a1 as $x)
    $b1[$x['sight_id']] = $x['location'];

$b2 =array();
foreach($a2 as $x)
    $b2[$x['sight_id']] = $x['location'];

Calculate the differences and intersection:

$c_intersect = array_intersect_key($b1,$b2);
$c_1 = array_diff_key($b1,$b2);
$c_2 = array_diff_key($b2,$b1);

Convert arrays back to your format:

$intersect_array = array();
foreach($c_intersect as $i=>$v)
    $intersect_array[] = array('sight_id'=>$i,'location'=>$v);

$only_a1 = array();
foreach($c_1 as $i=>$v)
    $only_a1[] = array('sight_id'=>$i,'location'=>$v);

$only_a2 = array();
foreach($c_2 as $i=>$v)
    $only_a2[] = array('sight_id'=>$i,'location'=>$v);

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