This looks mostly good thank you for working on it!

Another side effect of removing addslashes() is that it will not escape the backslash character anymore. This is a bit less obvious to test because it is actually fine in the case of a single backslash in a password. For example, if your password is:

my\password

Then either of these work fine:

define( 'DB_PASSWORD', 'my\\password' );

define( 'DB_PASSWORD', 'my\password' );

In the first case, PHP sees \ which it interprets as an escaped backslash (a single \ at runtime). In the second case, \p is not a recognized escape sequence in single-quoted strings, so both the \ and p are treated as literals, giving the same result. However, if your password has two backslashes in it like:

my\\password

Then the new code doesn't work, because it will insert:

define( 'DB_PASSWORD', 'my\\password' );

When it needs to be this to work properly:

define( 'DB_PASSWORD', 'my\\\\password' );

https://www.php.net/manual/en/language.types.string.php#language.types.string.syntax.single

To specify a literal single quote, escape it with a backslash (). To specify a literal backslash, double it (\). All other instances of backslash will be treated as a literal backslash: this means that the other escape sequences you might be used to, such as \r or \n, will be output literally as specified rather than having any special meaning.

So to keep the existing behavior while avoiding the issue with double-quotes (which addslashes() unnecessarily escapes), I think the change needs to be:

if ( is_string( $value ) ) {
  $value = str_replace( '\\', '\\\\', $value );  // Escape backslashes first
  $value = str_replace( "'", "\\'", $value );    // Then escape single quotes
  return $value;
}

With a test for a password with \\ that gets escaped properly. So probably something like:

When I run `wp config create --skip-check --dbname=somedb --dbuser=someuser --dbpass='my\\password'`
    Then the wp-config.php file should contain:
      """
      define( 'DB_PASSWORD', 'my\\\\password' )
      """

if ( is_string( $value ) ) {
$value = str_replace( '\', '\\', $value ); // Escape backslashes first
$value = str_replace( "'", "\'", $value ); // Then escape single quotes
return $value;
}

This seems working with the following complex passwords as well:
p\@(ss){w0r?d/><}"!Wi\th"Doub\leQuotes = p\\@(ss){w0r?d/><}"!Wi\\th"Doub\\leQuotes
my\password = my\\password
my\password = my\\\\password
p'\'@(ss){w0r?d/><}'!W\ith'Sing\leQuotes = p\'\\\'@(ss){w0r?d/><}\'!W\\ith\'Sing\\leQuotes
p\@(ss){w0r?d/><}"!Wi\th"Doub\leQuotes\"""\\\" = p\\@(ss){w0r?d/><}"!Wi\\th"Doub\\leQuotes\\"""\\\\"

Great! Can you add the suggested test for the backslashes as well (Just so we verify it works and then track it going forward)? Then I think this is good.

^ Just to re-trigger workflows :-)

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

grazie 😊