The class SoapClient was readily available in PHP 5.x, but not in PHP 7.
We must edit the file php.ini to enable SoapClient (and other API's).
Locate php.ini
On my computer it is C:\xampp\php\php.ini (small wheel like symbol on the icon).
Open php.ini with a text editor like NotePad or WordPad
Locate the lines, and remove the semicolons (;) in the beginning of the lines (; means "the rest of the line is a comment)
extension=php_soap.dll
extension=php_sockets.dll
extension=php_sqlite3.dll
;extension=php_sybase_ct.dll
;extension=php_tidy.dll
extension=php_xmlrpc.dll
extension=php_xsl.dll
Save the file php.ini
Re-start Apache from the XAMPP Control Panel