PHP : Strings and pattern - String basics

String Basics

Strings can be defined using one of several methods. Most commonly, you will encapsulate them in single quotes or double quotes. Unlike some other languages, these two methods behave quite differently: single quotes represent “simple strings,” where almost all characters are used literally. Double quotes, on the other hand, encapsulate “complex strings” that allow for special escape sequences (for example, to insert special characters) and for variable substitution, which makes it possible to embed the value of a variable directly in a string, without the need for any special operator.

Escape sequences are sometimes called control characters and take the form of a backslash (\) followed by one or more characters. Perhaps the most common escape sequence is the newline character \n. In the following example, we use hex and octal notation to display an asterisk:

echo "\x2a";

echo "\052";

Variable Interpolation

Variables can be embedded directly inside a double-quote string by simply typing their name. For example:

$who = "World";

echo "Hello $who\n"; // Shows "Hello World" followed by a newline

echo ’Hello $who\n’; // Shows "Hello $who\n"

Clearly, this “simple” syntax won’t work in those situations in which the name of the variable you want to interpolated is positioned in such a way inside the string that the parser wouldn’t be able to parse its name in the way you intend it to. In these cases, you can encapsulate the variable’s name in braces:

$me = ’Davey’;

$names = array (’Smith’, ’Jones’, ’Jackson’);

echo "There cannot be more than two {$me}s!";

echo "Citation: {$names[1]}[1987]";

In the first example above, the braces help us append a hard-coded letter “s” to the value of $me—without them, the parser w oul d be looking for the variable $mes, which, obviously, does not exist. In the second example, if the braces were not available, the parser would interpret our input as $names[1][1987], which is clearly not what we intended.

Escaping Literal Values

All three string-definition syntax feature a set of several characters that require escaping in order to be interpreted as literals.

When using single-quote strings, single quote characters can be escaped using a backslash:

echo ’This is \’my\’ string’;

A similar set of escaping rules apply to double-quote strings, where double quote characters and dollar sign can also be escaped by prefixing them with a backslash:

$a = 10;

echo "The value of \$a is \"$a\".";

Backslashes themselves can be escaped in both cases using the same technique:

echo "Here’s an escaped backslash: - \ -";

Note that you cannot escape a brace—therefore, i f you need the literal string {$ to be printed out, you need to escape the dollar sign in order to prevent the parser from interpreting the sequence as an attempt to interpolate a variable:

echo "Here’s a literal brace + dollar sign: {\$";

Heredoc strings provide the same escaping mechanisms as double-quote strings, with the exception that you do not need to escape double quote characters, since they have no semantic value.

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