Description:
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When performing a walk on dot1dTpFdbAddress (.1.3.6.1.2.1.17.4.3.1.1) with an HP Procurve switch (it's a bridge between the port and the mac address table), I believe the PHP SNMP module is interpreting the response incorrectly when a backslash is in the OID.

I'm not sure how the dot1dTpFdbAddress 'names' are created or what exactly they represent, but in this case, it appears if the 'name' includes a backslash, PHP is assigning the name as the value.  I'm guessing it's an incorrect regex.

This is the first time I've run into this issue, so I think it's rare.  I've tried various output options (quick_print, enum_print, output_format and valueretrieval), but the value doesn't change.

I've tried both PHP versions 7.0.27 and 7.2.1

Test script:
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# Command line snmpwalk command
% snmpwalk -v 2c -c public switch1 .1.3.6.1.2.1.17.4.3.1.1
BRIDGE-MIB::dot1dTpFdbAddress.'lbmP\=' = STRING: 6c:62:6d:50:5c:3d
BRIDGE-MIB::dot1dTpFdbAddress.'l.....' = STRING: 6c:f3:7f:c6:88:d6
BRIDGE-MIB::dot1dTpFdbAddress.'pM{&..' = STRING: 70:4d:7b:26:e3:4
BRIDGE-MIB::dot1dTpFdbAddress.'pZ....' = STRING: 70:5a:f:12:a2:11

# PHP either OOP or procedural
$snmp = new \SNMP(SNMP::VERSION_2c, 'switch1', 'public');
$snmp->walk('.1.3.6.1.2.1.17.4.3.1.1');
# or
snmp2_real_walk('switch1', 'public', '.1.3.6.1.2.1.17.4.3.1.1');

[SNMPv2-SMI::mib-2.17.4.3.1.1.108.98.109.80.92.61] => STRING: "lbmP\\="
[SNMPv2-SMI::mib-2.17.4.3.1.1.108.243.127.198.136.214] => Hex-STRING: 6C F3 7F C6 88 D6 
[SNMPv2-SMI::mib-2.17.4.3.1.1.112.77.123.38.227.4] => Hex-STRING: 70 4D 7B 26 E3 04 
[SNMPv2-SMI::mib-2.17.4.3.1.1.112.90.15.18.162.17] => Hex-STRING: 70 5A 0F 12 A2 11 


Expected result:
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The first output in the example should return a MAC address, not lbmP\\=