Source

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

public function connect() {
		if ( isset( $this->options['ssl'] ) && $this->options['ssl'] && function_exists( 'ftp_ssl_connect' ) ) {
			$this->link = @ftp_ssl_connect( $this->options['hostname'], $this->options['port'], FS_CONNECT_TIMEOUT );
		} else {
			$this->link = @ftp_connect( $this->options['hostname'], $this->options['port'], FS_CONNECT_TIMEOUT );
		}

		if ( ! $this->link ) {
			$this->errors->add(
				'connect',
				sprintf(
					/* translators: %s: hostname:port */
					__( 'Failed to connect to FTP Server %s' ),
					$this->options['hostname'] . ':' . $this->options['port']
				)
			);

			return false;
		}

		if ( ! @ftp_login( $this->link, $this->options['username'], $this->options['password'] ) ) {
			$this->errors->add(
				'auth',
				sprintf(
					/* translators: %s: Username. */
					__( 'Username/Password incorrect for %s' ),
					$this->options['username']
				)
			);

			return false;
		}

		// Set the connection to use Passive FTP.
		ftp_pasv( $this->link, true );

		if ( @ftp_get_option( $this->link, FTP_TIMEOUT_SEC ) < FS_TIMEOUT ) {
			@ftp_set_option( $this->link, FTP_TIMEOUT_SEC, FS_TIMEOUT );
		}

		return true;
	}