PHP: different quotes?

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 = '[email protected]';
$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 = [email protected]

$sql2 would be SELECT user_id FROM users WHERE email = $email

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.

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.