Programming PHP, 2nd Edition

Quoting String Constants

There are three ways to write a literal string in your program: using single quotes, double quotes, and the here document (heredoc) format derived from the Unix shell. These methods differ in whether they recognize special escape sequences that let you encode other characters or interpolate variables.

The general rule is to use the least powerful quoting mechanism necessary. In practice, this means that you should use single-quoted strings unless you need to include escape sequences or interpolate variables, in which case you should use double-quoted strings. If you want a string that spans many lines, use a heredoc.

Variable Interpolation

When you define a string literal using double quotes or a heredoc, the string is subject to variable interpolation . Interpolation is the process of replacing variable names in the string with the values of those variables. There are two ways to interpolate variables into strings—the simple way and the complex way.

The simple way is to put the variable name in a double-quoted string or heredoc:

    $who = 'Kilroy';
    $where = 'here';
    echo "$who was $where";
    Kilroy was here

The complex way is to surround the variable being interpolated with curly braces. This method can be used either to disambiguate or to interpolate array lookups. The classic use of curly braces is to separate the variable name from surrounding text:

    $n = 12;
    echo "You are the {$n}th person";
    You are the 12th person

Without the curly braces, PHP would try to print the value of the $nth variable.

Unlike in some shell environments, in PHP strings are not repeatedly processed for interpolation. Instead, any interpolations in a double-quoted string are processed, then the result is used as the value of the string:

    $bar = 'this is not printed';
    $foo = '$bar';     // single quotes
    print("$foo");
    $bar

Single-Quoted Strings

Single-quoted strings do not interpolate variables. Thus, the variable name in the following string is not expanded because the string literal in which it occurs is single-quoted :

    $name = 'Fred';
    $str  = 'Hello, $name';     // single-quoted
    echo $str;
    Hello, $name

The only escape sequences that work in single-quoted strings are \', which puts a single quote in a single-quoted string, and \\, which puts a backslash in a single-quoted string. Any other occurrence of a backslash is interpreted simply as a backslash:

    $name = 'Tim O\'Reilly';    // escaped single quote
    echo $name;
    $path = 'C:\\WINDOWS';      // escaped backslash
    echo $path;
    $nope = '\n';               // not an escape
    echo $nope;
    Tim O'Reilly
    C:\WINDOWS
    \n

Double-Quoted Strings

Double-quoted strings interpolate variables and expand the many PHP escape sequences. Table 4-1 lists the escape sequences recognized by PHP in double-quoted strings .

Table 4-1. Escape sequences in double-quoted strings

Escape sequence	Character represented
\"	Double quotes
\n	Newline
\r	Carriage return
\t	Tab
\\	Backslash
\$	Dollar sign
\{	Left brace
\}	Right brace
\[	Left bracket
\]	Right bracket
\0 through \777	ASCII character represented by octal value
\x0 through \xFF	ASCII character represented by hex value

If an unknown escape sequence (i.e., a backslash followed by a character that is not one of those in Table 4-1) is found in a double-quoted string literal, it is ignored (if you have the warning level E_NOTICE set, a warning is generated for such unknown escape sequences):

    $str = "What is \c this?";      // unknown escape sequence
    echo $str ;
    What is \c this?

Here Documents

You can easily put multiline strings into your program with a heredoc, as follows:

    $clerihew = <<< End_Of_Quote
    Sir Humphrey Davy
    Abominated gravy.
    He lived in the odium
    Of having discovered sodium.
    End_Of_Quote;
    echo $clerihew;
    Sir Humphrey Davy
    Abominated gravy.
    He lived in the odium
    Of having discovered sodium.

The <<< Identifier tells the PHP parser that you’re writing a heredoc. There must be a space after the <<< and before the identifier. You get to pick the identifier. The next line starts the text being quoted by the heredoc, which continues until it reaches a line that consists of nothing but the identifier.

As a special case, you can put a semicolon after the terminating identifier to end the statement, as shown in the previous code. If you are using a heredoc in a more complex expression, you need to continue the expression on the next line, as shown here:

    printf(<<< Template
    %s is %d years old.
    Template
    , "Fred", 35);

Single and double quotes in a heredoc are passed through:

    $dialogue = <<< No_More
    "It's not going to happen!" she fumed.
    He raised an eyebrow.  "Want to bet?"
    No_More;
    echo $dialogue;
    "It's not going to happen!" she fumed.
    He raised an eyebrow.  "Want to bet?"

Whitespace in a heredoc is also preserved:

    $ws = <<< Enough
      boo
      hoo

    Enough;
    // $ws = "  boo\n  hoo\n";

The newline before the trailing terminator is removed, so these two assignments are identical:

    $s = 'Foo';
    // same as
    $s = <<< End_of_pointless_heredoc
    Foo
    End_of_pointless_heredoc;

If you want a newline to end your heredoc-quoted string, you’ll need to add an extra one yourself:

    $s = <<< End
    Foo

    End;