The legacy mongo PHP extension was the original driver for connecting PHP to MongoDB. It was officially deprecated in 2015 and removed from active PECL support after PHP 5.6. If you are running PHP 7.0 or later, the mongo extension will not compile — you must use the modern mongodb extension instead.

This guide is provided for administrators who must maintain legacy PHP 5.x applications that cannot yet be migrated. If you have any choice in the matter, skip to the migration section below.

Legacy mongo Extension: Compatibility Table

PHP Version	mongo extension	mongodb extension	Recommendation
PHP 5.3 – 5.6	✅ Supported (last release: 1.6.16)	✅ Supported	Migrate to mongodb now
PHP 7.0 – 7.4	❌ Not supported (fails to compile)	✅ Supported	Use mongodb extension
PHP 8.0 – 8.3	❌ Not supported	✅ Supported	Use mongodb extension

Installing the Legacy mongo Extension (PHP 5.x Only)

Step 1 — Install Prerequisites

# CentOS 6 / CentOS 7 with PHP 5.x
yum install php-pear php-devel openssl-devel gcc make -y

Step 2 — Install the mongo Extension via PECL

# Install the last known-good version of the legacy mongo driver
pecl install mongo-1.6.16

Specifying version 1.6.16 is important — it is the final stable release and the only one that cleanly compiles against PHP 5.x. Running pecl install mongo without a version may attempt a newer, incompatible release.

Step 3 — Enable the Extension

# Add to php.ini
echo "extension=mongo.so" >> /etc/php.ini

# Or create a dedicated drop-in config (cleaner approach)
echo "extension=mongo.so" > /etc/php.d/mongo.ini

Step 4 — Restart Apache

# CentOS 6 (SysV init)
service httpd restart

# CentOS 7+ (systemd)
systemctl restart httpd

Step 5 — Verify

php -m | grep -i mongo

# Expected output:
# mongo

Migrating from mongo to mongodb (Strongly Recommended)

The legacy mongo extension uses a completely different API from the modern mongodb extension. You cannot simply swap the extension and expect your code to work — the classes and method names have changed. Here is what changes:

Legacy (mongo)	Modern (mongodb + library)
new Mongo()	new MongoDBClient()
new MongoClient()	new MongoDBClient()
$db->collection->find()	$db->collection->find() (similar)
new MongoId($id)	new MongoDBBSONObjectId($id)
new MongoDate()	new MongoDBBSONUTCDateTime()
MongoCursor	MongoDBDriverCursor

Migration Steps

1. Upgrade your server to PHP 7.4 or 8.x
2. Install the modern mongodb PECL extension (pecl install mongodb)
3. Install the MongoDB PHP Library: composer require mongodb/mongodb
4. Update your application code to use the new MongoDBClient API
5. Replace all legacy classes (MongoId, MongoDate, etc.) with their MongoDBBSON* equivalents
6. Run your test suite and fix any remaining compatibility issues

MongoDB provides an official upgrade guide in the PHP documentation covering the full API differences.