Source

File: wp-admin/includes/class-wp-filesystem-ftpext.php

public function put_contents( $file, $contents, $mode = false ) {
		$tempfile   = wp_tempnam( $file );
		$temphandle = fopen( $tempfile, 'wb+' );

		if ( ! $temphandle ) {
			unlink( $tempfile );
			return false;
		}

		mbstring_binary_safe_encoding();

		$data_length   = strlen( $contents );
		$bytes_written = fwrite( $temphandle, $contents );

		reset_mbstring_encoding();

		if ( $data_length !== $bytes_written ) {
			fclose( $temphandle );
			unlink( $tempfile );
			return false;
		}

		fseek( $temphandle, 0 ); // Skip back to the start of the file being written to.

		$ret = ftp_fput( $this->link, $file, $temphandle, FTP_BINARY );

		fclose( $temphandle );
		unlink( $tempfile );

		$this->chmod( $file, $mode );

		return $ret;
	}