How to replace a nth occurrence in a string

Question

I need a simple and fast solution to replace nth occurrence (placeholder) in a string.

For example, nth question mark in sql query should be replaced with a provide

Answer 1

Here is the function you asked for:

$subject = "SELECT uid FROM users WHERE uid = ? or username = ?";

function str_replace_nth($search, $replace, $subject, $nth)
{
    $found = preg_match_all('/'.preg_quote($search).'/', $subject, $matches, PREG_OFFSET_CAPTURE);
    if (false !== $found && $found > $nth) {
        return substr_replace($subject, $replace, $matches[0][$nth][1], strlen($search));
    }
    return $subject;
}

echo str_replace_nth('?', 'username', $subject, 1);

Note: $nth is a zero based index!

But I'll recommend to use something like the following to replace the placeholders:

$subject = "SELECT uid FROM users WHERE uid = ? or username = ?";

$args = array('1', 'steve');
echo vsprintf(str_replace('?', '%s', $subject), $args);