Expressions And Operators: String Concatenation

The binary operator . creates a string that is the concatenation of the left-hand operand and the right-hand operand, in that order. If either or both operands have types other than string, their values are converted to type string. Consider the following example:

<?hh // strict

namespace Hack\UserDocumentation\ExpAndOps\StringConcatenation\Examples\Point;

class Point {
  private float $x;
  private float $y;

  public function __construct(num $x = 0, num $y = 0) {
    $this->x = (float)$x;
    $this->y = (float)$y;
  }

  public function __toString(): string {
    return '('.$this->x.','.$this->y.')';
  }
  // ...
}

<<__EntryPoint>>
function main(): void {
  $p1 = new Point(20, 30);
  echo $p1."\n"; // implicit call to __toString()
}
Output
(20,30)

As $p1 designates an object, the expression $p1 . "\n" causes the method __toString to be called on that object, which returns the string "(20,30)", the concatenation of the three single-quoted string literals and the two floats. That string in turn is concatenated with the double-quoted string containing a newline, and the resulting string is written to standard output. Here are some more examples, which all involve conversion to string:

-10 . \NAN               // string result with value "-10NAN"
\INF . "2e+5"            // string result with value "INF2e+5"
true . null              // string result with value "1"