php strings

A string (or “string-literal”) is any number of characters enclosed within a pair of either single (') or double (") quotation marks.

echo "This is a php string.\n";
echo 'So is this, though we'll tend to prefer double-quotes (read more below).\n';
echo "\n";
echo "Remember that spaces are just regular ol' characters (not 'nothing'), ";
echo "and that the empty-string is a string (not 'nothing').\n";
echo "The length of five spaces: strlen(\"     \") = ", strlen("     ");
echo "The length of the empty string: strlen(\"\") = ", strlen("");
echo "The length of a string consisting of just the newline character: strlen(\"\\n\") = ", strlen("\n");

Note that unlike many languages, string-literals can contain contain newlines:

<?php
  echo "hello
        there
        everybody!";
  echo '
    -- Dr. Nick
         
   ';
     ?>

Rule of thumb: Use double-quoted strings most of the time, in your program; if printing/creating strings of html, let it be html whose attribute-values contain single-quoted strings (since html lets you use either double- or single-quotes, around attribute-values).
If using double-quotes, the and you want to include a double-quote in your string-literal, precede that character with a backslash \". (This case is rare, if you are following the preceding rule-of-thumb!) You can also embed other special characters, like newlines, by using \n, so "hi\nbye" is a six-character string, not seven. This is probably what you expect, from most other programming languages.
Just to keep things odd: double-quoted literals allow most standard quoted characters except for \' so "hi\'bye" is a seven-character string, not six!
If using single-quotes, the and you want to include a single-quote in your string-literal, precede that character with a backslash \', sensibly enough.
However, note that no other characters can be quoted, in single-quoted strings: 'hi\nbye' and 'hi\"bye' are both seven-character strings, not six! (And, variable-substitution, mentioned below, doesn't happen with single-quotes strings.)
So: No matter what type of string you're using, don't try to quote the other type of quote-marks.
Furthermore: If a string-literals delimited with double-quotes contains variables (a dollar-sign), then the variable gets replaced with its value!
This “interpolation”¹ occurs (only) within double-quoted strings, not single-quoted. This is feature of php, to make string-generation easy.
Note: I suggest using double-quotes for most php strings, by default. For HTML, we'll tend to use single-quotes

String examples:

$str = "How are you?";
$first_name = "John";
$str = "How are you " . $first_name . "?";
$str = "How are you $first_name?";  // alternately
$date = "1/01/2010";  // This is a string of course, not a "date type", nor division.
$str2 = "I said, \"how are you?\"";
// To use a variable followed by literal-chars that aren't part of the variable name:
$str3 = "Curly braces are as cute as you are, ${first_name}y-poo!";

here-doc strings

Actually, they come in two varieties: double-quoted, and single-quoted:

PHP's echo-mode vs. PHP-mode

PHP files themselves, outside of “<?php … ?>”, are implicit echo statements!

¹ A similar term is “transclusion”, including one document inside another, like Wikipedia-markup calls it. ↩

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