php - Convert Multidimensional Array to Single -

i using yii2 php framework , have query in accessing users in db:

$allusersquery = new query; $allusersquery->select(['_id'])->from('user')->where([ 'parent' => new \mongoid($session['company_owner']) ]); $allusers = $allusersquery->all();  

when var_dump $allusers array, gives me output:

array (size=5)   0 =>      array (size=1)       '_id' =>          object(mongoid)[147]           public '$id' => string '55d5a227650fc90c35000044' (length=24)   1 =>      array (size=1)       '_id' =>          object(mongoid)[148]           public '$id' => string '55d5a22a650fc90c35000047' (length=24)   2 =>      array (size=1)       '_id' =>          object(mongoid)[149]           public '$id' => string '55d5a22a650fc90c3500004a' (length=24)   3 =>      array (size=1)       '_id' =>          object(mongoid)[150]           public '$id' => string '55d5a22b650fc90c3500004d' (length=24)   4 =>      array (size=1)       '_id' =>          object(mongoid)[151]           public '$id' => string '55d5a22b650fc90c35000050' (length=24) 

it's multidimensional array. have searched , tried several solutions give me result:

array (size=1)   '_id' =>      object(mongoid)[147]       public '$id' => string '55d5a227650fc90c35000044' (length=24) 

here few solutions i've tried failed work:
1.

function array_flatten($allusers) {      if (!is_array($allusers)) {          return false;      }      $allusersresult = array();      foreach ($allusers $key => $value) {          if (is_array($value)) {              $allusersresult = array_merge($allusersresult, array_flatten($value));          }          else {            $allusersresult[$key] = $value;          }      }      return $allusersresult;  }  

2.

array_reduce($allusers, 'array_merge', array()); 

3.

$allusers = $allusers[0]; 

my expected output is:

array('value1', 'value2', 'value3'); 

how make work?

you can iterate through array , collect data.
have edited function:

function array_flatten($allusers) {      if (!is_array($allusers)) { return false; }       $allusersresult = array();      foreach ($allusers $key => $value) {          if (!is_array($value)) { return false; }           $allusersresult[] = $value["_id"]->{'$id'};     }      return $allusersresult;  }