POMO — PO/MO Translation File Handling

Last updated: March 4, 2026

WordPress gettext-based internationalization subsystem for reading, writing, and managing PO and MO translation files.

Since: 2.8.0 Source: wp-includes/pomo/

Components

Component	Description
class-translation-entry.md	Encapsulates a single translatable string
class-translations.md	Base class: a set of translation entries with headers
class-gettext-translations.md	Gettext-aware translations with plural form parsing
class-noop-translations.md	No-op implementation (passthrough, no actual translation)
class-mo.md	MO (Machine Object) binary file reader/writer
class-po.md	PO (Portable Object) text file reader/writer
class-plural-forms.md	Plural-Forms expression parser and evaluator
streams.md	Stream reader classes for binary file I/O

Architecture

Translations (2.8.0)
    ├── entries[], headers[]
    ├── translate(), translate_plural()
    └── add_entry(), merge_with()

Gettext_Translations (2.8.0) extends Translations
    ├── Plural-Forms header parsing
    ├── make_plural_form_function() → Plural_Forms
    └── make_headers() from raw translation string

MO (2.8.0) extends Gettext_Translations
    ├── import_from_file() / import_from_reader()
    ├── export() / export_to_file()
    └── Binary MO format: magic, revision, string tables

PO (2.8.0) extends Gettext_Translations
    ├── import_from_file() — line-by-line PO parser
    ├── export() / export_to_file()
    └── poify() / unpoify() — PO string escaping

NOOP_Translations (2.8.0)
    └── Same interface as Translations, all methods are no-ops

Translation_Entry (2.8.0)
    ├── singular, plural, context, translations[]
    ├── key() — generates lookup key (contextx04singular)
    └── merge_with() — merges flags, references, comments

Plural_Forms (4.9.0)
    ├── Shunting-yard parser → Reverse Polish Notation
    ├── execute() — stack-based RPN evaluator
    └── get() — cached plural form lookup

POMO_Reader → POMO_FileReader (file handle I/O)
           → POMO_StringReader (in-memory string I/O)
               → POMO_CachedFileReader (file read into memory)
                   → POMO_CachedIntFileReader

Key Concepts

Translation Lookup

Translations are keyed by a combination of context and singular string. The key format is:

Without context: singular
With context: contextx04singular (EOT character as separator)

Line endings are normalized to n in keys.

MO File Format

MO files are binary, containing:

Magic number (0x950412de) — also determines byte order
Revision — only revision 0 is supported
String count and table offsets
Original strings table — length/offset pairs
Translation strings table — length/offset pairs
Hash table (optional, skipped on read)
String data — null-terminated strings

Context is separated by x04, plural forms by x00.

PO File Format

PO files are text-based with entries like:

# translator comment
#. extracted comment
#: file.php:42
#, php-format
msgctxt "context"
msgid "singular"
msgid_plural "plural"
msgstr[0] "translation"
msgstr[1] "translations"

Plural Forms

The Plural-Forms header (e.g., nplurals=2; plural=n != 1;) is parsed by the Plural_Forms class using the shunting-yard algorithm to convert the C-like expression to RPN, then evaluated with a stack-based interpreter. Results are cached per-number.

Constants

Constant	Value	Source
`PO_MAX_LINE_LEN`	`79`	`po.php`

File Map

File	Classes
`entry.php`	`Translation_Entry`
`translations.php`	`Translations`, `Gettext_Translations`, `NOOP_Translations`
`mo.php`	`MO`
`po.php`	`PO`
`plural-forms.php`	`Plural_Forms`
`streams.php`	`POMO_Reader`, `POMO_FileReader`, `POMO_StringReader`, `POMO_CachedFileReader`, `POMO_CachedIntFileReader`

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