PHP array function array_key_exists() finds whether the array key exists

$a=array("a"=>"Dog","b"=>"Cat");
if (array_key_exists("a",$a))
 {
echo "Key exists!";
}
else
{
echo "Key does not exist!";
}
?>

Output: Key exists!

Example 2:

$a=array("a"=>"Dog","b"=>"Cat");
if (array_key_exists("c",$a))
{
echo "Key exists!";
}
else
{
echo "Key does not exist!";
}
?>

Output: Key does not exist!

Example 3:

$a=array("Dog",Cat");
if (array_key_exists(0,$a))
{
echo "Key exists!";
}
else
 {
echo "Key does not exist!";
}
?>

output: Key exists!