WordPress implementation of PHP sprintf() with filters.

function wp_sprintf( $pattern, ...$args ) {
	$len       = strlen( $pattern );
	$start     = 0;
	$result    = '';
	$arg_index = 0;
	while ( $len > $start ) {
		// Last character: append and break.
		if ( strlen( $pattern ) - 1 == $start ) {
			$result .= substr( $pattern, -1 );
			break;
		}

		// Literal %: append and continue.
		if ( '%%' === substr( $pattern, $start, 2 ) ) {
			$start  += 2;
			$result .= '%';
			continue;
		}

		// Get fragment before next %.
		$end = strpos( $pattern, '%', $start + 1 );
		if ( false === $end ) {
			$end = $len;
		}
		$fragment = substr( $pattern, $start, $end - $start );

		// Fragment has a specifier.
		if ( '%' === $pattern[ $start ] ) {
			// Find numbered arguments or take the next one in order.
			if ( preg_match( '/^%(d+)$/', $fragment, $matches ) ) {
				$index    = $matches[1] - 1; // 0-based array vs 1-based sprintf() arguments.
				$arg      = isset( $args[ $index ] ) ? $args[ $index ] : '';
				$fragment = str_replace( "%{$matches[1]}$", '%', $fragment );
			} else {
				$arg = isset( $args[ $arg_index ] ) ? $args[ $arg_index ] : '';
				++$arg_index;
			}

			/**
			 * Filters a fragment from the pattern passed to wp_sprintf().
			 *
			 * If the fragment is unchanged, then sprintf() will be run on the fragment.
			 *
			 * @since 2.5.0
			 *
			 * @param string $fragment A fragment from the pattern.
			 * @param string $arg      The argument.
			 */
			$_fragment = apply_filters( 'wp_sprintf', $fragment, $arg );
			if ( $_fragment != $fragment ) {
				$fragment = $_fragment;
			} else {
				$fragment = sprintf( $fragment, (string) $arg );
			}
		}

		// Append to result and move to next fragment.
		$result .= $fragment;
		$start   = $end;
	}

	return $result;
}