The PHP function array_shift will remove the first element from the array and return its value. Note that changes to the array are made in the original array, so when we call the array again, it will no longer have its first element.

The PHP function preg_match compares a given string $subject with a given regular expression $pattern.

PHP has several functions that can be used with regular expressions; regular expressions are commonly referred to as regex in programming.

The array_search function searches the array for a value that corresponds to the given value. If the given value is found in the array, it will return its key

The PHP array_push function adds one or more values to the end of an array. As you can see in the description, the array is passed by reference, meaning that this function will modify the original array.