PHP get index into new array

you can use array_keys() or key() with a foreach loop for this(DEMO):

$newarr = [];

foreach($jsonarr as $value)
{
    //$key = array_keys($value)[0];
    $key = key($value);
    $newarr[] = ['key' => $key, 'id' => $value[$key]['id']];
}
var_dump($newarr);

Output:

array(2) {
  [0]=>
  array(2) {
    ["key"]=>
    int(11111111)
    ["id"]=>
    string(17) "val_somevalue5555"
  }
  [1]=>
  array(2) {
    ["key"]=>
    int(2222222)
    ["id"]=>
    string(15) "val_somevalue25"
  }
}

Edit: With the updated json, you can use the following way, using 2 foreach loops (DEMO):

$newarr = [];

foreach($jsonarr as $json)
{
    foreach($json as $key => $value)
    {
        $newarr[] = ['key' => $key, 'id' => $value['id']];
    }
}

PHP supports a slightly different foreach syntax that extracts both the array key and the array value:

foreach ( $jsonarr as $key => $value ) {
    $newarr[] = ['key' => $key, 'id' => $value];
}

Use this if you need the key ("11111111" and "2222222" in your example).

<?php
$json = '[{
    "11111111": {
        "id": "val_somevalue5555"
    }
},
{
    "2222222": {
        "id": "val_somevalue25"
    }
}
]';

$jsonarr = json_decode($json, true);

$newarr = [];
foreach($jsonarr as $key => $value) {
        $newarr[] = ['key' => key($value), 'id' => current($value)['id']];
}

foreach($newarr as $key) {
    echo 'key '.$key['key'] . PHP_EOL;
    echo 'id '.$key['id'] . PHP_EOL;
}

If you remove what looks like embedded components in the $json string (otherwise it won't parse) then var_export the output of json_decode() you'll get this:

array (
   0 => array (
      11111111 => array (
          'id' => 'val_somevalue5555',
      ),
   ),
   1 => array (
      2222222 => array (
          'id' => 'val_somevalue25',
      ),
   ),
)

You have a double-nested array, hence...

foreach ($jsonarr as $obj) {
   foreach ($obj as $name=>$value) {
      print "$name = $value[id]\n";
      break;
   }
}

or you can reference the elements directly:

print $jsonarr[0]['11111111']['id'];

You can create an array of the keys of an existing array using the array_keys() function

http://php.net/manual/en/function.array-keys.php

If you don't want the keys in a separate array, and instead just want to access them directly, when you are doing a 'foreach' loop of an array, you can choose to assign a variable to the current key by doing

foreach($jsonarr as $key => $value){...}  

Because your original array is actually multidimensional (each $key has a $value that is also stored as an array of "id": "value") - this means taking one more step to get the value of key 'id':

foreach($jsonarr as $key => $value){     
 $newarray[] = ['key' => $key, 'id' => $value['id'];
}

First, you are not accessing the deep enough before iterating.

If you call var_export($jsonarr); you will see:

array (                                 // an indexed array of subarrays
  0 =>
  array (                               // an associative array of subarrays, access via [0] syntax (or a foreach loop that only iterates once)
    11111111 =>                         // this is the subarray's key that you want
    array (
      'id' => 'val_somevalue5555',      // this is the value you seek from the id element of 1111111's subarray
      'customer' => 
      array (
        32312 => 
        array (
          'name' => 'jane doe',
        ),
      ),
    ),
    2222222 =>                         // this is the subarray's key that you want
    array (
      'id' => 'val_somevalue25',      // this is the value you seek from the id element of 2222222's subarray
      'customer' => 
      array (
        32312234 => 
        array (
          'name' => 'jane doe',
        ),
      ),
    ),
  ),
)

Code: (Demo)

$jsonarr = json_decode($json, true);
$result=[];
//                     vvvv-avoid a function call (key()) on each iteration by declaring here
foreach($jsonarr[0] as $key=>$subarray){
    //          ^^^-drill down into the first level (use another foreach loop if there may be more than one)
    $result[]=['key'=>$key,'id'=>$subarray['id']];
}
var_export($result);

Output:

array (
  0 => 
  array (
    'key' => 11111111,
    'id' => 'val_somevalue5555',
  ),
  1 => 
  array (
    'key' => 2222222,
    'id' => 'val_somevalue25',
  ),
)

p.s. If $jsonarr has more than one element in its first level, you should use a foreach() loop like this:

foreach($jsonarr as $array1){
    foreach($array1 as $key=>$array2){
        $result[]=['key'=>$key,'id'=>$array2['id']];
    }
}