In PHP, every script has a 30-second time limit to complete, which can cause server timeouts if API requests, database transactions, and data processing take too long. To measure script execution time in sending an SMS with Twilio Programmable Messaging, developers need to use the microtime() function to record start and end times of the process, calculate the time difference, and display the results. By doing so, they can make more performant decisions about their application's performance and avoid leaving users waiting for too long.