NoSQL databases are non-relational databases that are designed to store and retrieve large volumes of unstructured and semi-structured data. These databases provide flexible schemas, horizontal scalability, and high availability. PHP is a popular programming language that is widely used for building web applications.

There are several NoSQL databases that can be used with PHP. Some of the popular choices include:

1. MongoDB: MongoDB is a document-oriented NoSQL database that stores data in JSON-like documents. It provides a flexible schema and supports complex queries. PHP has an official MongoDB driver that allows you to connect to MongoDB and perform CRUD operations.

2. CouchDB: CouchDB is a distributed NoSQL database that stores data in JSON-like documents. It provides a RESTful API for accessing and manipulating data. PHP has a library called “php-couchdb” that provides a simple way to interact with CouchDB.

3. Redis: Redis is an in-memory data structure store that can be used as a database, cache, and message broker. It supports various data structures like strings, lists, sets, and hashes. PHP has a library called “php-redis” that provides a way to interact with Redis.

4. Cassandra: Cassandra is a distributed NoSQL database that is designed to handle large amounts of data across many commodity servers. It provides high availability and fault tolerance. PHP has a library called “phpcassa” that provides a way to connect to Cassandra and perform CRUD operations.

5. Neo4j: Neo4j is a graph database that stores structured data in the form of nodes, relationships, and properties. It provides efficient querying of highly connected data. PHP has a library called “NeoClient” that allows you to interact with Neo4j.

These are just a few examples of NoSQL databases that can be used with PHP. There are many other NoSQL databases and PHP libraries available, so you can choose the one that best fits your needs.