Description:
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It seems to be impossible to replace a key in associative array with another associative array : array_splice always add a '0'  or next unused numeric key for incoming array... This would seem to be a bug.

Reproduce code:
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<?php
        $a = array("a" => "AA", "b" => "BB", "c" => "CC");
        $a2 = $a;
        $z = array("z" => array("z1"=>"ZZ1","z2"=>"ZZ2"), "x" =>"XX");
        
// 1 try to replace "b"=>"BB" by contents of $z
        array_splice($a, 1, 1, $z);
        print_r($a);
// produces : Array ( [a] => AA [0] => Array ( [z1] => ZZ1 [z2] => ZZ2 )
[1] => XX [c] => CC ) 
        
// 2 try to replace "b"=>"BB" by contents of $z as in manual
        array_splice($a, 1, 1, array($z));
        print_r($a);
// produces: Array ( [a] => AA [0] => Array ( [z] => Array ( [z1] => ZZ1 [z2] => ZZ2 ) [x] => XX ) [1] => XX [c] => CC )
?>

Expected result:
----------------
// Should be :
// Array ( [a] => AA [z] => Array ( [z1] => ZZ1 [z2] => ZZ2 ) [1] => XX
[c] => CC )