Source

File: wp-admin/includes/file.php

function wp_tempnam( $filename = '', $dir = '' ) {
	if ( empty( $dir ) ) {
		$dir = get_temp_dir();
	}

	if ( empty( $filename ) || in_array( $filename, array( '.', '/', '\\' ), true ) ) {
		$filename = uniqid();
	}

	// Use the basename of the given file without the extension as the name for the temporary directory.
	$temp_filename = basename( $filename );
	$temp_filename = preg_replace( '|\.[^.]*$|', '', $temp_filename );

	// If the folder is falsey, use its parent directory name instead.
	if ( ! $temp_filename ) {
		return wp_tempnam( dirname( $filename ), $dir );
	}

	// Suffix some random data to avoid filename conflicts.
	$temp_filename .= '-' . wp_generate_password( 6, false );
	$temp_filename .= '.tmp';
	$temp_filename  = $dir . wp_unique_filename( $dir, $temp_filename );

	$fp = @fopen( $temp_filename, 'x' );
	if ( ! $fp && is_writable( $dir ) && file_exists( $temp_filename ) ) {
		return wp_tempnam( $filename, $dir );
	}
	if ( $fp ) {
		fclose( $fp );
	}

	return $temp_filename;
}