Are there differences between /, /* */ and #?

Question

Considering how important it is to comment on a code, I came up with this question.

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I realized there are several ways to comment lines/parts of code:

• //
• /* */
• #

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Example:

// comentando uma linha

/* comentando
   um intervalo
   do código */

# comentando uma linha

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Doubts:

• There are other ways to comment?
• Taking these definitions from the example, there are more differences between them?

Answer 1

The essential thing is this:

• The // and the # are identical in terms of operation;

• The // and the /* are completely different in terms of operation, and each has a specific use scenario:

  The // or # are for line comments. Everything that comes after them is ignored, until you find a line break.

  Already, the /* is a block comment which can end: 1) before the end of the line; or 2) different line from where it started. In both cases what determines the end of the comment is the sequence */;

• There is a third situation, the __halt_compiler, that is not comment, but it also causes the code to be ignored from that point on (it can be used to "ignore" a longer comment, or even have a special function to reuse the source file).

Otherwise, no differences at all.

Notes:

• You can’t nest block comments.

  /* abc /* 123 */ def */
  

  in the above case, def is no longer commented, will give error. The first */ it is worth. (The fact is that the second /* is ignored).

• There’s a technique for "keying" very curious block comments, which is this one (credits to the PHP comment section):

  When you define a block like this, just modify a bar to "count" it or not:

  //*
      Bloco "chaveável" com várias linhas.
      Em princípio, são duas linhas com "comment de linha"
      Basta modificar UMA barra, que vira um "comment de bloco"
  // */
  

  Except for the first bar:

  /*
      Bloco "chaveável" com várias linhas.
      Em princípio, são duas linhas com "comment de linha"
      Basta modificar UMA barra, que vira um "comment de bloco"
  // */
  

• It has a type of comment that is used in documentation tools (not part of the language itself) which is the /**. In practice is a normal block comment /* followed by an asterisk (although, a special token was adopted for this case in the parser, the T_DOC_COMMENT).

  For more details, see this post:

  What’s the difference between "/*" and "/**" comments in PHP?