What Characters Are Valid for PDO Placeholders in PHP?

Valid PDO Placeholder Characters

Working with placeholders in PHP with PDO can be tricky when it comes to selecting valid characters. To delve into the details, let's first examine the example code provided:

$calories = 150;
$colour = 'red';
$sth = $dbh->prepare('SELECT name, colour, calories
    FROM fruit
    WHERE calories < :calories AND colour = :colour');
$sth->bindParam(':calories', $calories, PDO::PARAM_INT);
$sth->bindParam(':colour', $colour, PDO::PARAM_STR, 12);
$sth->execute();

In this example, we're using the placeholder ":colour." What characters are considered valid for placeholders? The answer lies in the source code. Upon inspection, we discover the following regular expression:

BINDCHR     = [:][a-zA-Z0-9_]+;

This means that a placeholder must consist of the following elements:

• An optional leading colon (:)
• At least one alphanumeric character (a-z, A-Z, 0-9)
• An optional trailing underscore (_)

Therefore, valid placeholder characters include:

• All lowercase letters (a-z)
• All uppercase letters (A-Z)
• All numbers (0-9)
• The underscore character (_)

Hyphens (-), as mentioned in the question, are not valid characters. Alphanumeric characters with underscores provide a convenient and comprehensive range of options for placeholder names.

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