php: The difference between require_once and require

Difference:

require_once will only load the same file once, while require will load it multiple times.

I believe that novices like me still have some doubts. So we guessed: If 456.php introduced 123.php, now the file test_require.php introduces "456.php" and "123.php". If the require method is used, then in test_require.php, 123.php will be introduced twice, and using the require_once method 123.php will be introduced once. So let’s do the experiment!

1. require method

The content of 123.php is as follows:

<?php
	echo "123\n";

The content of 456.php is as follows:

##

<?php
<span style="white-space:pre">	</span>require("123.php");
<span style="white-space:pre">	</span>echo "456\n";

The content of test_require.php is as follows:

<?php
	require("123.php");
	require("456.php");

Run test_require.php and get the following results:

2. require_once method

123.php content remains unchanged. 456.php and test_require.php change the require_once method. The content is as follows:

##

//456.php
<?php
	require_once "123.php";
	echo "456\n";

//test_require.php
<?php
	require_once "123.php";
	require_once "456.php";

Run test_require.php, the results are as follows:

3. Mixed use error

If the require method is used in 456.php and the require_once method is used, then require_once will be invalid. The codes of the three files are as follows:

//123.php
<?php
	echo "123\n";
//456.php
The running results are as follows: 


Summary: as As we guessed, the require_once method will not load the same file multiple times. But mixing it with require will make the require_once method invalid. 
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