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Programming PHP, 3rd Edition by Peter MacIntyre, Kevin Tatroe, Rasmus Lerdorf

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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.

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 simpler of the two ways 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 other way is to surround the variable being interpolated with curly braces. Using this syntax ensures the correct variable is interpolated. The classic use of curly braces is to disambiguate 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 first and 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 = <<< EndOfQuote
Sir Humphrey Davy
Abominated gravy.
He lived in the odium
Of having discovered sodium.
EndOfQuote;
echo $clerihew;
Sir Humphrey Davy
Abominated gravy.
He lived in the odium
Of having discovered sodium.

The <<< identifier token 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 = <<< NoMore
"It's not going to happen!" she fumed.
He raised an eyebrow. "Want to bet?"
NoMore;
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";

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

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

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

$s = <<< End
Foo

End;

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