$packet_out = 4;
On the serial wire: 0x34 (which is ASCII code for character 4)

$packet_out = '4';
On the serial wire: 0x34 (which is ASCII code for character 4)

$packet_out = 0x65;
On the serial wire: 0x31 0x30 0x31 (which, bizarrely, is ASCII code for '101' which is a string of the decimal ASCII code for 'e' which has the ASCII code 0x65)