PHP array merging: Analysis of the differences between + operator, array_merge, and array_merge_recursive

$a = array(
 'a',
);
$b = array(
 'u',
);
$c = $a + $b;
var_dump($c);

Output:

$a = array(
 66=>'a',
);
$b = array(
 60=>'u',
 66=>'c'
);
 $c = $a + $b;
var_dump($c);

Output:

$a = array(
 1=>'a',
 2=>'b',
 'c'=>'c',
 'd'= >'d',
);
$b = array(
 1=>'u',
 3=>'v',
 'c'=>'w',
 'd'=> ;'x',
 'y'=>'y',
 60=>'z',
);
$c = $a + $b;
var_dump($c);
?>

Output:

$a = array(
 'a',
);
$b = array(
 'u',
);
$c = array_merge($a, $b);
var_dump($ c);

Output:

$a = array(
 66=>'a',
);
$b = array(
 60=>'u',
 66=>'c'
);
 $c = array_merge($a, $b);
var_dump($c);

Output:

$a = array(
 1=>'a',
 2=>'b',
 'c'=>'c',
 'd'=>'d' ,
);
$b = array(
 1=>'u',
 3=>'v',
 'c'=>'w',
 'd'=>'x',
 'y'=>'y',
 60=>'z',
);
$c = array_merge($a, $b);
var_dump($c);

Output:

$a = array(
 'a',
);
$b = array(
 'u',
);
$c = array_merge_recursive($a, $b);
var_dump($ c);

Output:

$a = array(
 'a',
);
$b = array(
 'u',
);
$c = array_merge_recursive($a, $b);
var_dump($ c);

Output:

$a = array(
 1=>'a',
 2=>'b',
 'c'=>'c',
 'd'=>'d' ,
);
$b = array(
 1=>'u',
 3=>'v',
 'c'=>'w',
 'd'=>'x',
 'y'=>'y',
 60=>'z',
);
$c = array_merge_recursive($a, $b);
var_dump($c);

Output:

array(8) { [0]=> string(1) "a" [1]=> string(1) "b" ["c"]=> array(2) { [0]=> string(1) "c" [1]=> string(1) "w" } ["d"]=> array(2) { [0]=> string(1) "d" [1]=> string(1) "x" } [2]=> string(1) "u" [3]=> string(1) "v" ["y"]=> string(1) "y" [4]=> string(1) "z" }