What PHP 8.1 changes for WordPress plugins

These are the functions, constants and settings that PHP 8.1 removes, deprecates or no longer ships, with what to use instead. A removed function or constant stops a plugin with a fatal error when that code runs. A deprecated one keeps working and logs a notice.

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Deprecated

strftime(), gmstrftime(). Use date(), wp_date() or IntlDateFormatter.
date_sunrise(), date_sunset(). Use date_sun_info().
mhash(), mhash_keygen_s2k(), mhash_count(), mhash_get_block_size(), mhash_get_hash_name(). Use the hash functions.
strptime(). Use date_parse_from_format().
odbc_result_all(). Build the table from odbc_fetch_array().
FILTER_SANITIZE_STRING, FILTER_SANITIZE_STRIPPED. Use sanitize_text_field() or htmlspecialchars().
FILE_BINARY, FILE_TEXT. The flag never had an effect. Remove it.
auto_detect_line_endings, filter.default. The setting is deprecated.

Bumpcheck also checks syntax changes, such as array offsets with curly braces and constructors named after their class. The full list is on the plugin page.