Fix non-runnable code examples (undefined variables, syntax errors)

[lacatoire]

This fixes a batch of code examples that cannot run as written:

• undefined variables (a missing $ sigil, or a variable name that does not
  match the one declared just above);
• a few syntax errors: a missing ;, a stray . that turns a subtraction
  into string concatenation, a = instead of =>, an unterminated string;
• a Cyrillic homoglyph (С, U+0421) used in a type hint instead of the Latin
  C class;
• a couple of examples instantiating or calling the wrong class/method name.

Each change only touches the example so it parses and runs as intended.