how to get only keys from array in php example ID , NAME , ADDRESS

Example 1: array key value php

<?php
$age=array("Peter"=>"35","Ben"=>"37","Joe"=>"43");
foreach($age as $x=>$x_value)
  {
  echo "Key=" . $x . ", Value=" . $x_value;
  echo "<br>";
  }
?>

Example 2: php take out 2 level array key value

foreach($bigArray as $array){    
    foreach($array as $key=>$value){
        echo $key;
    }
}


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