obfuscate or encrypt some plain text data in PHP

for simple obfuscation use strtr() - Translate certain characters:
string strtr ( string $str , string $from , string $to )

to encode in php:

$readable='This is a special test string ABC123 ([+,-!#$%&*])';    $unreadable=strtr($readable,' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡ˆ‰Š‹ŒŽ‘’“”•–—˜™š›œžŸ ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ'                           ,'¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡ˆ‰Š‹ŒŽ‘’“”•–—˜™š›œžŸ '                      );print $unreadable; //outputs: "ÕéêôAêôAâAôñæäêâíAõæôõAôõóêïèAÂÃIJ³´A©Ü¬­®¢¤¥¦§«Þª"

to decode in php:

$unreadable='ÕéêôAêôAâAôñæäêâíAõæôõAôõóêïèAÂÃIJ³´A©Ü¬­®¢¤¥¦§«Þª';$readable=strtr($unreadable,'¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡ˆ‰Š‹ŒŽ‘’“”•–—˜™š›œžŸ '                           ,' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡ˆ‰Š‹ŒŽ‘’“”•–—˜™š›œžŸ ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ'               );print $readable; //outputs: "This is a special test string ABC123 ([+,-!#$%&*])"

you can easily replicate this logic in the DB if necessary (without looping): Using a Table of Numbers, by Erland Sommarskog

How about base64 encoding? We use to use that to make SMS messages in our SMS Gateway DB unreadable by the developers.

There are a few options.

If you want very strong, you could look into mcrypt.

But if it's only so working developers cant read the text without some work to actually do it. Then you could just BASE64 encode it or uuencode it