- var_dump: documentation ( source)
<?php
$a1=['0'=>'my key should be a (numeric) string'];
$a2=[0=>'my key should be an int'];
$a3= new stdClass;
$a3->{"0"} = 'my key should be a (numeric) string';
$a3=(array)$a3;
foreach($a1 as $key=>$val){echo $val.': ';var_dump($key);}
foreach($a2 as $key=>$val){echo $val.': ';var_dump($key);}
foreach($a3 as $key=>$val){echo $val.': ';var_dump($key);}
var_dump($a1,$a2,$a3);