How can the presence of outdated or deprecated PHP functions impact the functionality and performance of a PHP application, as highlighted in the thread?

The presence of outdated or deprecated PHP functions can impact the functionality and performance of a PHP application by causing compatibility issues with newer PHP versions, security vulnerabilities, and potential bugs. To solve this issue, it is recommended to replace deprecated functions with their modern equivalents or alternative solutions provided by the PHP community.

// Example of replacing deprecated function with its modern equivalent
// Deprecated function: mysql_connect()
// Modern equivalent: mysqli_connect()

// Deprecated code
$conn = mysql_connect('localhost', 'username', 'password');

// Updated code
$conn = mysqli_connect('localhost', 'username', 'password');