How can I decrypt a password hash in PHP?

Bcrypt is a one-way hashing algorithm, you can't decrypt hashes. Use password_verify to check whether a password matches the stored hash:

<?php
// See the password_hash() example to see where this came from.
$hash = '$2y$07$BCryptRequires22Chrcte/VlQH0piJtjXl.0t1XkA8pw9dMXTpOq';

if (password_verify('rasmuslerdorf', $hash)) {
    echo 'Password is valid!';
} else {
    echo 'Invalid password.';
}

In your case, run the SQL query using only the username:

$sql_script = 'SELECT * FROM USERS WHERE username=?';

And do the password validation in PHP using a code that is similar to the example above.

The way you are constructing the query is very dangerous. If you don't parameterize the input properly, the code will be vulnerable to SQL injection attacks. See this Stack Overflow answer on how to prevent SQL injection.

The passwords cannot be decrypted as will makes a vulnerability for users. So, you can simply use password_verify() method to compare the passwords.

if(password_verify($upass, $userRow['user_pass'])){
     //code for redirecting to login screen }

where, $upass is password entered by user and $userRow['user_pass'] is user_pass field in database which is encrypted by password_hash() function.

I need to decrypt a password. The password is crypted with password_hash function.

$password = 'examplepassword';
$crypted = password_hash($password, PASSWORD_DEFAULT);

Its not clear to me if you need password_verify, or you are trying to gain unauthorized access to the application or database. Other have talked about password_verify, so here's how you could gain unauthorized access. Its what bad guys often do when they try to gain access to a system.

First, create a list of plain text passwords. A plain text list can be found in a number of places due to the massive data breaches from companies like Adobe. Sort the list and then take the top 10,000 or 100,000 or so.

Second, create a list of digested passwords. Simply encrypt or hash the password. Based on your code above, it does not look like a salt is being used (or its a fixed salt). This makes the attack very easy.

Third, for each digested password in the list, perform a select in an attempt to find a user who is using the password:

$sql_script = 'select * from USERS where password="'.$digested_password.'"'

Fourth, profit.

So, rather than picking a user and trying to reverse their password, the bad guy picks a common password and tries to find a user who is using it. Odds are on the bad guy's side...

Because the bad guy does these things, it would behove you to not let users choose common passwords. In this case, take a look at ProCheck, EnFilter or Hyppocrates (et al). They are filtering libraries that reject bad passwords. ProCheck achieves very high compression, and can digest multi-million word password lists into a 30KB data file.