I'm looking for a list of all locales and their short codes for a PHP application I am writing. Is there much variation in this data between platforms?
Also, if I am developing an international application, can I just support one version of English or are there significant differences in English across the world?
Here's a pretty exhaustive list of Culture Codes. As far as I can tell, they don't vary between programming languages since it's an RFC standard. As for English, I think if you support either the generic en or possibly the en-US then you should be just fine.
From http://www.w3.org/International/articles/language-tags/
"Language tag syntax is defined by the IETF's BCP 47. BCP stands for 'Best Current Practice', and is a persistent name for a series of RFCs whose numbers change as they are updated. The latest RFC describing language tag syntax is RFC 5646, Tags for the Identification of Languages, and it obsoletes the older RFCs 4646, 3066 and 1766.
You used to find subtags by consulting the lists of codes in various ISO standards, but now you can find all subtags in the IANA Language Subtag Registry."
AFAIK most locale-aware applications (that are written by professionals) abide by this standard. It isn't just something somebody threw together and that different people interpret differently.
I'd strongly suggest you investigate the internationalization features of your particular development language, as you'll probably end up reinventing the wheel if you don't.
The importance of locales is that your environment/os can provide formatting functionality for all installed locales even if you don't know about them when you write your application. My Windows 7 system has 211 locales installed (listed below), so you wouldn't likely write any custom code or translation specific to this many locales.
The most important ting for various versions of English is in formatting numbers and dates. Other differences are significant to the extent that you want and able to cater to specific variations.
af-ZA
am-ET
ar-AE
ar-BH
ar-DZ
ar-EG
ar-IQ
ar-JO
ar-KW
ar-LB
ar-LY
ar-MA
arn-CL
ar-OM
ar-QA
ar-SA
ar-SY
ar-TN
ar-YE
as-IN
az-Cyrl-AZ
az-Latn-AZ
ba-RU
be-BY
bg-BG
bn-BD
bn-IN
bo-CN
br-FR
bs-Cyrl-BA
bs-Latn-BA
ca-ES
co-FR
cs-CZ
cy-GB
da-DK
de-AT
de-CH
de-DE
de-LI
de-LU
dsb-DE
dv-MV
el-GR
en-029
en-AU
en-BZ
en-CA
en-GB
en-IE
en-IN
en-JM
en-MY
en-NZ
en-PH
en-SG
en-TT
en-US
en-ZA
en-ZW
es-AR
es-BO
es-CL
es-CO
es-CR
es-DO
es-EC
es-ES
es-GT
es-HN
es-MX
es-NI
es-PA
es-PE
es-PR
es-PY
es-SV
es-US
es-UY
es-VE
et-EE
eu-ES
fa-IR
fi-FI
fil-PH
fo-FO
fr-BE
fr-CA
fr-CH
fr-FR
fr-LU
fr-MC
fy-NL
ga-IE
gd-GB
gl-ES
gsw-FR
gu-IN
ha-Latn-NG
he-IL
hi-IN
hr-BA
hr-HR
hsb-DE
hu-HU
hy-AM
id-ID
ig-NG
ii-CN
is-IS
it-CH
it-IT
iu-Cans-CA
iu-Latn-CA
ja-JP
ka-GE
kk-KZ
kl-GL
km-KH
kn-IN
kok-IN
ko-KR
ky-KG
lb-LU
lo-LA
lt-LT
lv-LV
mi-NZ
mk-MK
ml-IN
mn-MN
mn-Mong-CN
moh-CA
mr-IN
ms-BN
ms-MY
mt-MT
nb-NO
ne-NP
nl-BE
nl-NL
nn-NO
nso-ZA
oc-FR
or-IN
pa-IN
pl-PL
prs-AF
ps-AF
pt-BR
pt-PT
qut-GT
quz-BO
quz-EC
quz-PE
rm-CH
ro-RO
ru-RU
rw-RW
sah-RU
sa-IN
se-FI
se-NO
se-SE
si-LK
sk-SK
sl-SI
sma-NO
sma-SE
smj-NO
smj-SE
smn-FI
sms-FI
sq-AL
sr-Cyrl-BA
sr-Cyrl-CS
sr-Cyrl-ME
sr-Cyrl-RS
sr-Latn-BA
sr-Latn-CS
sr-Latn-ME
sr-Latn-RS
sv-FI
sv-SE
sw-KE
syr-SY
ta-IN
te-IN
tg-Cyrl-TJ
th-TH
tk-TM
tn-ZA
tr-TR
tt-RU
tzm-Latn-DZ
ug-CN
uk-UA
ur-PK
uz-Cyrl-UZ
uz-Latn-UZ
vi-VN
wo-SN
xh-ZA
yo-NG
zh-CN
zh-HK
zh-MO
zh-SG
zh-TW
zu-ZA