A 'pit' in PHP

　Talk about a problem that you are likely to encounter at work - a quote from foreach

<span>$arr</span> = <span>range</span>(1,3<span>);
</span><span>//</span><span>[1,2,3]</span>

<span>foreach</span>(<span>$arr</span> <span>as</span> &<span>$val</span><span>) {
}

</span><span>foreach</span>(<span>$arr</span> <span>as</span> <span>$val</span><span>) {
}
</span><span>print_r</span>(<span>$arr</span><span>);
 </span>

　What does the above code output? The amazing thing is that it is as follows. I encountered this once at work. It took me a long time to figure out the reason, but I just found a solution. There are two solutions to this problem:

<span>Array</span><span>
(
    [</span>0] => 1<span>
    [</span>1] => 2<span>
    [</span>2] => 2<span>
)</span>

The following two methods can solve the above problem:

<span>//</span><span>方法1</span>
<span>foreach</span> (<span>$arr</span> <span>as</span> &<span>$value</span><span>) {
}
</span><span>unset</span>(<span>$value</span><span>);
</span><span>foreach</span> (<span>$arr</span> <span>as</span> <span>$value</span><span>) {
}
</span><span>print_r</span>(<span>$arr</span><span>);
</span><span>//</span><span>[1,2,3]

//方法2</span>
<span>foreach</span> (<span>$arr</span> <span>as</span> &<span>$value</span><span>) {
}
</span><span>foreach</span> (<span>$arr</span> <span>as</span> <span>$val</span><span>) {
}
</span><span>print_r</span>(<span>$arr</span><span>);
</span><span>//</span><span>[1,2,3]
//方法3</span>
<span>foreach</span> (<span>$arr</span> <span>as</span> &<span>$value</span><span>) {
}
</span><span>foreach</span> (<span>$arr</span> <span>as</span> &<span>$value</span><span>) {
}
</span><span>print_r</span>(<span>$arr</span><span>);
</span><span>//</span><span>[1,2,3]</span>

Method 1 can also be seen in the official manual http://php.net/manual/en/control-structures.foreach.php. There is a special tip in the article to remind you of this

Why are Method 2 and Method 3 OK? You will know after reading the following. In fact, it is the reference in "causing trouble". The &$value in foreach is a pointer variable pointing to an element in the array. Let's look at the following first. This It’s easy to understand. tmp is a reference to var, pointing to the storage space of var. When tmp changes, var also changes

<span>$var</span> = 123<span>;
</span><span>$tmp</span> = &<span>$var</span><span>;
</span><span>$tmp</span> = 200<span>;
</span><span>echo</span> <span>$var</span><span>;
</span><span>//</span><span>200</span>

Take a look at the stolen picture below (haha, the original link is posted below) to have a better understanding of the above

Okay, after this is ok, look at the following (I don’t know what software was used to draw the above picture, I can only draw it by hand, embarrassing~~), let’s look at the first foreach first:

Second foreach:

At this time, it is easy to know that in fact, in the second foreach loop, the value of the last element in the array is really changeable~~, from the first element to the second to last element. To solve this problem, it is very simple, cut off $ The relationship between value and the second foreach. Therefore, you can unset, change the variable name, or reset the pointer (method 3). Well, at this point, you can basically understand the problem at this point.

Okay, let’s take a look at the comments below the official document. It should be easy to understand

<?<span>php
</span><span>$arr1</span> = <span>array</span>("a" => 1, "b" => 2, "c" => 3<span>);
</span><span>$arr2</span> = <span>array</span>("x" => 4, "y" => 5, "z" => 6<span>);

</span><span>foreach</span> (<span>$arr1</span> <span>as</span> <span>$key</span> => &<span>$val</span><span>) {}
</span><span>foreach</span> (<span>$arr2</span> <span>as</span> <span>$key</span> => <span>$val</span><span>) {}

</span><span>var_dump</span>(<span>$arr1</span><span>);
</span><span>var_dump</span>(<span>$arr2</span><span>);
</span>?><span>

The output is</span>:
<span>array</span>(3) { ["a"]=> int(1) ["b"]=> int(2) ["c"]=> &int(6<span>) }
</span><span>array</span>(3) { ["x"]=> int(4) ["y"]=> int(5) ["z"]=> int(6) }

Reference article:

　http://www.cnblogs.com/CraryPrimitiveMan/p/4030748.html#3085766

http://www.jb51.net/article/39299.htm

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The above introduces a "pit" in PHP, including aspects of it. I hope it will be helpful to friends who are interested in PHP tutorials.