Source

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

public function connect() {
		if ( ! $this->ftp ) {
			return false;
		}

		$this->ftp->setTimeout( FS_CONNECT_TIMEOUT );

		if ( ! $this->ftp->SetServer( $this->options['hostname'], $this->options['port'] ) ) {
			$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 ( ! $this->ftp->connect() ) {
			$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 ( ! $this->ftp->login( $this->options['username'], $this->options['password'] ) ) {
			$this->errors->add(
				'auth',
				sprintf(
					/* translators: %s: Username. */
					__( 'Username/Password incorrect for %s' ),
					$this->options['username']
				)
			);

			return false;
		}

		$this->ftp->SetType( FTP_BINARY );
		$this->ftp->Passive( true );
		$this->ftp->setTimeout( FS_TIMEOUT );

		return true;
	}