Will PHP's unset() release memory at all?

1. How to check the current memory

Use memory_get_usage() function to view memory

2. Experiment

<?php

    function aa()
    {
    
    
        var_dump(memory_get_usage(false));  //int(22356464)
        for ($q=1;$q<1000;$q++){
    
    
            $a[] = $q;
        }
        var_dump(memory_get_usage(false));   //int(22393384)
        array_pop($a);
        var_dump(memory_get_usage(false));   //int(22393408)
        $b = $a;
        var_dump(memory_get_usage(false)); //int(22393408)
        unset($a);
        var_dump(memory_get_usage(false)); //int(22393384)
        unset($b);
        var_dump(memory_get_usage(false)); //int(22356464)
    }
    
    function dd()
    {
    
    
        $this->aa();
        var_dump('last:'.memory_get_usage(false)); // last:22356464
    }

You can see that this experiment is a method that calls another method to test the memory changes after the variable is unset.

1. The initial memory value is 22356464
2. 2 Create an array $a and fill it with 1000 values. At this time, the memory reaches 22393384, and the memory increases by 36860.
3. We remove a value in the array and look at the memory: 22393408, the memory has increased by 24.
4. Then assign $a to $b, then look at the memory: 22393408, there is no change in the memory.
5. After unset($a), the memory falls back to 22393384, a decrease of 24.
6. Then unset($b) again, this is the memory falling back to the initial value of 22356464
7. In the return dd() function, the memory is still the initial value 22356464

From this experiment, we can see that if $a has been assigned to other variables, when we unset the original variable $a, it will not release its original memory. In fact, when we assign $a to $b, we can find that the memory does not double the memory of $a. In fact, $a and $b are equivalent to labels, pointing to this pile of data. When we unset it In fact, $a just removes the "label" and does not point to this pile of data. If this pile of data is not labeled, it will be released. If there is such a $b that also points to this pile of data, it will not be released. .
So unset() can release memory, provided that this variable is not assigned to another variable