What best practices should be followed when defining variables in PHP?

When defining variables in PHP, it is best practice to use meaningful names that describe the data they hold. Variables should start with a letter or underscore, followed by letters, numbers, or underscores. Additionally, variables are case-sensitive in PHP, so be consistent with the casing throughout your code.

// Good practice: defining variables with meaningful names
$firstName = "John";
$lastName = "Doe";

// Avoid: using unclear variable names
$fn = "John";
$ln = "Doe";