if( isset( $array['index'] ) {
   //Do something
}


if( array_key_exists( 'index', $array ) {
   //Do something
}

$aTestArray = array();

echo "Before key is createdrn";
echo "isset:rn";
var_dump( isset( $aTestArray['TestKey'] ) );
echo "array_key_exists:rn";
var_dump( array_key_exists( 'TestKey', $aTestArray ) );
echo "rn";

$aTestArray['TestKey'] = NULL;
echo "Key is created, but set to NULLrn";
echo "isset:rn";
var_dump( isset( $aTestArray['TestKey'] ) );
echo "array_key_exists:rn";
var_dump( array_key_exists( 'TestKey', $aTestArray ) );
echo "rn";

$aTestArray['TestKey'] = 0;
echo "Key is created, and set to 0 (zero)rn";
echo "isset:rn";
var_dump( isset( $aTestArray['TestKey'] ) );
echo "array_key_exists:rn";
var_dump( array_key_exists( 'TestKey', $aTestArray ) );
echo "rn";

Before key is created
isset:
bool(false)
array_key_exists:
bool(false)

Key is created, but set to NULL
isset:
bool(false)
array_key_exists:
bool(true)

Key is created, and set to 0 (zero)
isset:
bool(true)
array_key_exists:
bool(true)

foreach( $array as $key => value )