String interpolation is a common feature in many programming languages such as Ruby, PHP, Perl, etc. It means to insert a string or replace a variable with its value. It makes string formatting and specifying contents more intuitive.^[1]

Examples

PHP

<?php
$str = <<<EOD
Example of string
spanning multiple lines
using heredoc syntax.
EOD;
 
class foo
{
    var $foo;
    var $bar;
 
    function foo()
    {
        $this->foo = 'Foo';
        $this->bar = array('Bar1', 'Bar2', 'Bar3');
    }
}
 
$foo = new foo();
$name = 'Jason';
 
echo <<<EOT
My name is "$name". I am printing some $foo->foo.
Now, I am printing some {$foo->bar[1]}.
This should print a capital 'A': \x41
EOT;
?>

The output will be:

My name is "Jason". I am printing some Foo.
Now, I am printing some Bar2.
This should print a capital 'A': A

Perl

#!/usr/bin/perl
use strict;
use warnings;
my $apples = 4;
print "I have $apples apples\n";

The output will be:

I have 4 apples

Ruby

apples = 4
puts "I have #{apples} apples"
# or
puts "I have %s apples" % apples

The output will be:

I have 4 apples

BOO

apples = 4
print("I have $(apples) apples")
// or
print("I have {0} apples" % apples)

The output will be:

I have 4 apples

CoffeeScript

apples = 4
console.log "I have #{apples} apples"

The output will be:

I have 4 apples

Python

apples = 4
print "I have %s apples" % apples

The output will be:

I have 4 apples

Security Issues

String Interpolation, like string concatenation, may lead to security problems. When failed to properly escape or filter user input data, system will expose to SQL Injection, Script Injection, XML External Entity Injection (XXE), and Cross Site Scripting (XSS) attacks.^[2]

An example of SQL Injection will be like this:

query = "SELECT x, y, z FROM Table WHERE id= '$id'

If id is replaced with "'; DELETE FROM Table WHERE = '", executing this query will wipe out all the data on the local machine.

See also

• String literal
• Concatenation
• Improper input validation

Notes