PHP Memcached Replace (Detailed Guide w/ Code Examples)

Use Case(s)

The Memcached::replace() function in PHP is used to replace the value of an existing item. It's frequently used when you need to update data that is stored in a Memcached server, but only if that key already exists. It's different from set() as it won't create a new key-value pair if the key doesn't exist.

Code Examples

Below is a simple example of how to use Memcached::replace(). This example assumes that a key "TestKey" already exists in the Memcached server.

<?php $mem = new Memcached(); $mem->addServer("localhost", 11211); $key = "TestKey"; $newValue = "New Value"; // Attempt to replace the value for the given key if ($mem->replace($key, $newValue)) { echo "Value was replaced successfully.\n"; } else { echo "Replacing value failed.\n"; } ?>

In this code, we first create an instance of the Memcached class and add a server. We then try to replace the value associated with "TestKey". If the operation is successful, we output a success message, otherwise we output a failure message.

Best Practices

  1. Always check if the key exists before calling replace(). Use the get() method to check if a key exists and handle it accordingly.
  2. When handling multiple servers, ensure consistency of data. Consider using consistent hashing or other techniques to distribute keys evenly among servers.

Common Mistakes

  1. Using replace() without ensuring the key already exists: Unlike set(), replace() won't create a new key-value pair if the key doesn't exist.
  2. Not handling errors: Always handle failures from replace() (like when a key is not found).


  1. What's the difference between Memcached::replace() and Memcached::set()?
  • Memcached::replace() only updates the value if the key already exists, whereas Memcached::set() will set the value whether the key exists or not.
  1. Can I use Memcached::replace() to change the expiration time of an existing item?
  • Yes, the Memcached::replace() method accepts an optional expiration time argument. If provided, this will update the expiration time of the item.

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