Understanding and Fixing the sprintf PHP Error: Unknown Format Specifier “w”



If you’ve ever encountered the error “unknown format specifier ‘w’” in PHP while using the sprintf function, you’re not alone. This common issue arises when sprintf encounters an unrecognized format specifier. Let’s dive into what causes this error and how to fix it.

What is sprintf?

The sprintf function in PHP is used to format a string according to a specified format. It takes a format string and a list of variables, returning a formatted string. Here are some common format specifiers:

  • %s – String
  • %d – Decimal number (integer)
  • %f – Floating-point number

The Problem

The error occurs when sprintf encounters an unknown format specifier. In our case, this was %w. For example:

$line = '<li>' . sprintf($text, $date, $time) . '</li>';

An error of type E_ERROR was caused in line 375 of the file /wp-content/plugins/wp-accessibility/wp-accessibility-stats.php. Error message: Uncaught ValueError: Unknown format specifier "w" in /wp-content/plugins/wp-accessibility/wp-accessibility-stats.php:376

If $text contains %w, PHP throws an error because it doesn’t recognize %w as a valid format specifier.

The Solution

The issue might arise if the format string contains a literal % that isn’t meant to be a format specifier. To include a literal % in the output string, you need to escape it using %%.

Here’s how you can fix the error by replacing % with %%:

  1. Identify the Issue: If $text contains % signs intended as literal characters, sprintf misinterprets them as format specifiers.
  2. Replace % with %%: Use str_replace to replace all % with %% in the format string.
$text = str_replace('%', '%%', $text);
$line = '<li>' . sprintf($text, $date, $time) . '</li>';

This ensures that sprintf treats % as a literal percentage sign and not a format specifier. By replacing % with %%, sprintf understands that %% should be a literal % and processes the other specifiers (%s for strings) without any errors.


When working with sprintf, always ensure that literal % characters are properly escaped as %%. This small adjustment can save you from frustrating errors and ensure your strings are formatted correctly.

By understanding the cause of the “unknown format specifier ‘w’” error and implementing this simple fix, you can confidently use sprintf in your PHP projects without running into unexpected issues.

Happy coding!


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