PHP returns after merging all the second level of $arr two-dimensional array

PHP returns all the second levels of the $arr two-dimensional array after merging them

array_merge() Merges the cells of one or more arrays, and the values ​​in one array are appended to the previous array. Returns the resulting array.
But it can only merge the first layer, but cannot merge the two-dimensional part of the second-level array
I wrote some code and it feels very bad. I hope experts can point out a better method

<?php
$arr=array(
 "0" => array(
 "44" => array("id" => 44,"name" => 'Nine Fish Picture '),
 "45" => array("id" => 45,"name" => 'Koi')
 ),
 "1" => array(
 "49" => array("id" => 49,"name" => 'Baifu Tu'),
 "50" => array("id" => 50,"name" => 'Bat')
 ),
 "2" => array(
 "40" => array("id" => 40,"name" => 'ancient money'),
 "41" => array( "id" => 41,"name" => 'Calabash')
 )
);
//Merge all the second levels of the $arr two-dimensional array and return
function array_merge_array($arr)
{
 $str="$"."son_arr=array_merge(";
 foreach($arr as $k => $v)
 {$str.='$arr['.$k.'],';}
 $str.=");";
 eval(str_replace(',)',')',$str));
 return $son_arr;
}
print_r(array_merge_array($arr));

/*
 Get the result
Array
(
 [0] => Array
 (
 [id] => 44
 [name] => Nine Fish Diagram
 )

 [1] => Array
 (
 [id ] => 45
 [name] => Koi
 )

 [2] => Array
 (
 [id] => 49
 [name] => Baifu Tu
 )

 [ 3] => Array
 (
 [id] => 50
 [name] => bat
 )

 [4] => Array
 (
 [id] => 40
 [name] = > Ancient Coin
 )

 [5] => Array
 (
 [id] => 41
 [name] => Gourd
 )

)

*/
?>