What is the underlying implementation principle of the in_array function in PHP?

In PHP, the in_array function is used to check if a value exists in an array. The underlying implementation is as follows:

1. Traversing the array: The in_array function first loops through every element in the array, starting from the first element and comparing them one by one.
2. Comparison value: For each element, the in_array function compares the array element with the value passed in using the strict (===) operator. The strict operator not only compares values, but also data types.
3. Return true if a matching element is found, or continue iterating through the array. If no matching element is found after iterating through the entire array, then return false.

It is important to note that the in_array function compares elements one by one without sorting the array. Therefore, its time complexity is O(n), where n is the length of the array. For large arrays, performance may be impacted. If frequent checks for element existence are needed, other data structures like hash tables or indexed arrays can be considered to improve performance.