Bug Report

• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior

Using WP CLI Nightly version: 2.8.0-alpha-751ac75 using completions yield a PHP Deprecated message in the middle of the command to be completed.

Error: PHP Deprecated: Creation of dynamic property WP_CLI\Completions::$cur_word is deprecated in phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Completions.php on line 25

Using https://github.com/wp-cli/wp-cli/blob/main/utils/wp-completion.bash

Here a screenshot: I tried typing "wp eva" + [TAB]

Describe how other contributors can replicate this bug

• Install wp cli nightly 2.8.0-alpha-751ac75
• download https://github.com/wp-cli/wp-cli/blob/main/utils/wp-completion.bash
• put the line in your .bash_profile: source /path/to/wp-completion.bash
• Try the completion by typing "wp [tab key]"