PHP: different quotes? - php

Question

What is the difference between the quotes " and ' ? What about `? Is there an error in using different quotes ' and " below?
$result = pg_query_params($dbconn,
'INSERT INTO users
(username, email, passhash_md5)
VALUES ($1, $2, $3)',
array($username, $email, $passhash_md5
)
$result = pg_query_params( $dbconn,
"SELECT user_id
FROM users
WHERE email = $1",
array( $email )
)

Answer 1

Variable-substitution isn't done when using single quotes ('), meaning that the values in your first example would literally be $1 $2 etc if it was a regular string and not passed on to a function that replaces them.
If you don't need variable-substitiution, it's better to stick with single quotes for performance reasons.
`` invokes the shell-engine and invokes it as an actual command, and returning the result, just like in perl. Hence, it has a completely different meaning.
examples:
$email = 'user#example.org';
$sql1 = "SELECT user_id FROM users WHERE email = $email";
$sql2 = 'SELECT user_id FROM users WHERE email = $email';
$sql1 would be SELECT user_id FROM users WHERE email = user#example.org
$sql2 would be SELECT user_id FROM users WHERE email = $email

Answer 2

Basically, " lets you embed variables like so:
<?php
$beer = 'Heineken';
echo "$beer's taste is great"; // works; "'" is an invalid character for variable names
echo "He drank some $beers"; // won't work; 's' is a valid character for variable names but the variable is "$beer"
echo "He drank some ${beer}s"; // works
echo "He drank some {$beer}s"; // works
?>
(From the php manual)
Using ' means that no checking for variables is done.
<?php
echo '$beer';
?>
Would output $beer.

Answer 3

The difference between single and double quoted strings is well explained in the PHP manual about Strings.
In your example, since you are using substitution variables such as $1 that mean something specific to pg_query_params and that you do not want PHP to interpret as variable names, you should use single quotes for your SQL query strings.