WordPress uses UTF-8 character encoding with MySQL/MariaDB collations optimized for Unicode.

Default Settings

Since WordPress 4.2 (2015):

// wp-config.php defaults
define( 'DB_CHARSET', 'utf8mb4' );
define( 'DB_COLLATE', '' );  // Uses database default

utf8mb4 vs utf8

WordPress requires utf8mb4 for full emoji and international character support.

Common Collations

WordPress default: utf8mb4_unicode_520_ci

Checking Current Settings

Database Level

SELECT @@character_set_database, @@collation_database;

Table Level

SELECT table_name, table_collation 
FROM information_schema.tables 
WHERE table_schema = 'your_database';

Column Level

SELECT column_name, character_set_name, collation_name
FROM information_schema.columns
WHERE table_schema = 'your_database'
  AND table_name = 'wp_posts';

Show Table Status

SHOW TABLE STATUS LIKE 'wp_posts';

WordPress Configuration

In wp-config.php

// Full 4-byte UTF-8 (recommended)
define( 'DB_CHARSET', 'utf8mb4' );

// Let WordPress choose collation
define( 'DB_COLLATE', '' );

// Or specify explicitly
define( 'DB_COLLATE', 'utf8mb4_unicode_520_ci' );

In $wpdb

global $wpdb;
$wpdb->charset;   // 'utf8mb4'
$wpdb->collate;   // 'utf8mb4_unicode_520_ci'

Converting Existing Tables

Check for Mixed Encodings

SELECT table_name, table_collation
FROM information_schema.tables
WHERE table_schema = 'your_database'
  AND table_collation NOT LIKE 'utf8mb4%';

Convert Table to utf8mb4

ALTER TABLE wp_posts 
CONVERT TO CHARACTER SET utf8mb4 
COLLATE utf8mb4_unicode_520_ci;

Convert All Tables

-- Generate ALTER statements
SELECT CONCAT(
    'ALTER TABLE `', table_name, '` ',
    'CONVERT TO CHARACTER SET utf8mb4 ',
    'COLLATE utf8mb4_unicode_520_ci;'
)
FROM information_schema.tables
WHERE table_schema = 'your_database'
  AND table_type = 'BASE TABLE';

Index Length Limits

The 767 Byte Limit

Older MySQL (pre-5.7) limits index key length to 767 bytes.

With utf8mb4 (4 bytes/char): 767 / 4 = 191 characters max

This is why you see indexes like:

KEY `meta_key` (`meta_key`(191))

Modern MySQL (5.7+)

With innodb_large_prefix=ON and ROW_FORMAT=DYNAMIC:

• Index limit: 3072 bytes
• 768 characters for utf8mb4

WordPress tables use ROW_FORMAT=Dynamic when available.

Common Issues

Emoji Broken

Cause: Tables still using utf8 (3-byte)
Fix: Convert to utf8mb4

"Specified key was too long"

Cause: Index exceeds byte limit
Fix:

• Use (191) prefix for varchar indexes
• Enable large prefix support
• Use ROW_FORMAT=DYNAMIC

Collation Mismatch Errors

Cause: Joining tables with different collations
Fix: Ensure all tables use same collation, or cast in query:

SELECT * FROM wp_posts p
JOIN wp_postmeta pm ON p.ID = pm.post_id
WHERE pm.meta_key COLLATE utf8mb4_unicode_520_ci = 'my_key';

Mojibake (Garbled Characters)

Cause: Double-encoding or wrong connection charset
Fix:

// Ensure connection uses utf8mb4
$wpdb->set_charset( $wpdb->dbh, 'utf8mb4' );

Best Practices

1. Use utf8mb4 for new installations
2. Use utf8mb4_unicode_520_ci collation
3. Set charset in wp-config.php before install
4. Check collation consistency across all tables
5. Backup before converting existing databases
6. Test emoji after conversion (📝✅🎉)

Checking Emoji Support

-- Insert test emoji
INSERT INTO wp_options (option_name, option_value, autoload) 
VALUES ('emoji_test', '🎉📝✅', 'no');

-- Verify it stored correctly
SELECT option_value FROM wp_options WHERE option_name = 'emoji_test';

-- Clean up
DELETE FROM wp_options WHERE option_name = 'emoji_test';

Server Requirements

• MySQL 5.5.3+ or MariaDB 5.5+
• innodb_file_format = Barracuda (for large indexes)
• innodb_large_prefix = ON (MySQL < 8.0)