blocks|key|506101|text|如果您没有声明一个新的数组，并且创建/更新数组的数据由于任何原因而失败，那么将来尝试使用该数组的任何代码都将E_FATAL，因为该数组不存在。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|506102|例如，如果未声明数组且未向其添加值，则foreach()将抛出错误。但是，如果数组只是为空，则不会发生错误，就像声明它时一样。|506103|entityMap^0|1I|7|0|J|9|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

If you don't declare a new array, and the data that creates / updates the array fails for any reason, then any future code that tries to use the array will <code>E_FATAL</code> because the array doesn't exist.

For example, <code>foreach()</code> will throw an error if the array was not declared and no values were added to it. However, no errors will occur if the array is simply empty, as would be the case had you declared it.

blocks|key|506179|text|Php是一种松散类型的语言。这是完全可以接受的。也就是说，真正的程序员总是声明他们的var。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|506180|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Php is a loosely typed language. It's perfectly acceptable. That being said, real programmers always declare their vars.

blocks|key|506250|text|想想那些追随你的程序员吧！如果只看到$arr[]+=+5，那么如果不阅读作用域中所有前面的代码，就不知道$arr可能是什么。显式的$arr+=+array()行清楚地说明了这一点。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|506251|entityMap^0|I|A|1G|4|1T|E|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

Think of the coders who come after you! If you just see <code>$arr[] = 5</code>, you have no idea what <code>$arr</code> might be without reading all the preceding code in the scope. The explicit <code>$arr = array()</code> line makes it clear.

blocks|key|506011|text|这只是一个很好的练习。假设您是在循环内附加到数组中(相当常见的做法)，但随后在所述循环之外访问数组。如果没有数组声明，如果没有进入循环，代码就会抛出错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|506012|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

it's just good practice. Let's say you were appending to your array inside a loop (fairly common practice), but then accessing the array outside of said loop. Without an array declaration, your code would throw errors if you never made it into the loop.

blocks|key|506302|text|我强烈建议在添加值之前声明数组。除了上面提到的一切之外，如果数组在循环中，您可能会无意中将元素推入数组。我只是观察到这造成了一个代价高昂的bug。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|506303|//Example+code++++
foreach+($mailboxes+as+$mailbox){
+++++++//loop+creating+email+list+to+get
+++++++foreach+($emails+as+$email){
++++++++++$arr[]+=+$email;
+++++++}
+++++++//loop+to+get+emails
+++++++foreach+($arr+as+$email){
+++++++//oops+now+we're+getting+other+peoples+emails
+++++++//in+other+mailboxes+because+we+didn't+initialize+the+array
+++++++}
}|code-block|syntax|javascript|506304|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I strongly recommend to declare the array before adding values. Besides everything mentioned above, if the array is inside a loop you might unintentionally push elements into your array. I just observed this creating a costly bug.

<pre><code>//Example code 
foreach ($mailboxes as $mailbox){
 //loop creating email list to get
 foreach ($emails as $email){
 $arr[] = $email;
 }
 //loop to get emails
 foreach ($arr as $email){
 //oops now we're getting other peoples emails
 //in other mailboxes because we didn't initialize the array
 }
}
</code></pre>

blocks|key|722496|text|在使用数组之前不声明数组确实会导致问题。我刚刚发现的一个经验是，我将此测试脚本命名为:+indextest.php?file=1STLSPGTGUS，它的工作方式与预期一致。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|722497|//indextest.php?file=1STLSPGTGUS
$path['templates']+++++=+'./mytemplates/';
$file['template']++++++=+'myindex.tpl.php';
$file['otherthing']++++++=+'otherthing';
$file['iamempty']++++=+'';

print+("path['templates']+=+"+.+$path['templates']+.+" ");
print+("file['template']+=+"+.+$file['template']+.+" ");
print+("file['otherthing']+=+"+.+$file['otherthing']+.+" ");
print+("file['iamempty']+=+"+.+$file['iamempty']+.+" ");

print+("file['file']+=+"+.+$file['file']+.+" ");//+should+give:+"Notice:+Undefined+index:+file"
print+("file+=+"+.+$file);//+should+give:+"Notice:+Undefined+index:+file"

//the+Output+is:
/*
path['templates']+=+./mytemplates/
file['template']+=+myindex.tpl.php
file['otherthing']+=+otherthing
file['iamempty']+=

Notice:+Undefined+index:+file+in+D:\Server\Apache24\htdocs\DeliverText\indextest.php+on+line+14
file['file']+=

Notice:+Array+to+string+conversion+in+D:\Server\Apache24\htdocs\DeliverText\indextest.php+on+line+15
file+=+Array
*/|code-block|syntax|javascript|722498|现在我只需要一个文件，来自我购买的另一个脚本，在我的顶部，我们可以看到数组$file的值是如何完全错误的，而数组$path是正常的："checkgroup.php“是有问题的。|722499|//indextest.php?file=1STLSPGTGUS
require_once($_SERVER['DOCUMENT_ROOT']."/IniConfig.php");
$access+=+"PUBLIC";
require_once(CONFPATH+.+"include_secure/checkgroup.php");
$path['templates']+++++=+'./mytemplates/';
$file['template']++++++=+'myindex.tpl.php';
$file['otherthing']++++++=+'otherthing.php';
$file['iamempty']++++=+'';

print+("path['templates']+=+"+.+$path['templates']+.+" ");
print+("file['template']+=+"+.+$file['template']+.+" ");
print+("file['otherthing']+=+"+.+$file['otherthing']+.+" ");
print+("file['iamempty']+=+"+.+$file['iamempty']+.+" ");

print+("file['file']+=+"+.+$file['file']+.+" ");
print+("file+=+"+.+$file);

//the+Output+is:
/*
path['templates']+=+./mytemplates/
file['template']+=+o
file['otherthing']+=+o
file['iamempty']+=+o
file['file']+=+o
file+=+oSTLSPGTGUS
*/|722500|之前初始化数组，那就没问题了！|722501|//indextest.php?file=1STLSPGTGUS
require_once($_SERVER['DOCUMENT_ROOT']."/IniConfig.php");
$access+=+"PUBLIC";
require_once(CONFPATH+.+"include_secure/checkgroup.php");

$path+=+array();
$file+=+array();

$path['templates']+++++=+'./mytemplates/';
$file['template']++++++=+'myindex.tpl.php';
$file['otherthing']++++++=+'otherthing.php';
$file['iamempty']++++=+'';

print+("path['templates']+=+"+.+$path['templates']+.+" ");
print+("file['template']+=+"+.+$file['template']+.+" ");
print+("file['otherthing']+=+"+.+$file['otherthing']+.+" ");
print+("file['iamempty']+=+"+.+$file['iamempty']+.+" ");

print+("file['file']+=+"+.+$file['file']+.+" ");
print+("file+=+"+.+$file);

//the+Output+is:
/*
path['templates']+=+./mytemplates/
file['template']+=+myindex.tpl.php
file['otherthing']+=+otherthing.php
file['iamempty']+=
file['file']+=
file+=+Array
*/|722502|这就是我如何认识到初始化变量的重要性，因为我们永远不知道以后会遇到什么问题，仅仅为了节省时间，我们最终可能会浪费更多的时间。我希望这会对像我这样不专业的人有所帮助。|722503|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

By not declaring an array before using it can really cause problems.
One experience I just found, I called this test script like this: indextest.php?file=1STLSPGTGUS
This works as expected.

<pre><code>//indextest.php?file=1STLSPGTGUS
$path['templates'] = './mytemplates/';
$file['template'] = 'myindex.tpl.php';
$file['otherthing'] = 'otherthing';
$file['iamempty'] = '';

print ("path['templates'] = " . $path['templates'] . "&lt;br&gt;");
print ("file['template'] = " . $file['template'] . "&lt;br&gt;");
print ("file['otherthing'] = " . $file['otherthing'] . "&lt;br&gt;");
print ("file['iamempty'] = " . $file['iamempty'] . "&lt;br&gt;");

print ("file['file'] = " . $file['file'] . "&lt;br&gt;");// should give: "Notice: Undefined index: file"
print ("file = " . $file);// should give: "Notice: Undefined index: file"

//the Output is:
/*
path['templates'] = ./mytemplates/
file['template'] = myindex.tpl.php
file['otherthing'] = otherthing
file['iamempty'] =

Notice: Undefined index: file in D:\Server\Apache24\htdocs\DeliverText\indextest.php on line 14
file['file'] =

Notice: Array to string conversion in D:\Server\Apache24\htdocs\DeliverText\indextest.php on line 15
file = Array
*/
</code></pre>

Now I will just require a file, from another script I bought, at the top of mine and we can see how values are completly wrong for the array $file while array $path is OK:
"checkgroup.php" is the guilty one.

<pre><code>//indextest.php?file=1STLSPGTGUS
require_once($_SERVER['DOCUMENT_ROOT']."/IniConfig.php");
$access = "PUBLIC";
require_once(CONFPATH . "include_secure/checkgroup.php");
$path['templates'] = './mytemplates/';
$file['template'] = 'myindex.tpl.php';
$file['otherthing'] = 'otherthing.php';
$file['iamempty'] = '';

print ("path['templates'] = " . $path['templates'] . "&lt;br&gt;");
print ("file['template'] = " . $file['template'] . "&lt;br&gt;");
print ("file['otherthing'] = " . $file['otherthing'] . "&lt;br&gt;");
print ("file['iamempty'] = " . $file['iamempty'] . "&lt;br&gt;");

print ("file['file'] = " . $file['file'] . "&lt;br&gt;");
print ("file = " . $file);

//the Output is:
/*
path['templates'] = ./mytemplates/
file['template'] = o
file['otherthing'] = o
file['iamempty'] = o
file['file'] = o
file = oSTLSPGTGUS
*/
</code></pre>

Initialising the array before, then no problem!

<pre><code>//indextest.php?file=1STLSPGTGUS
require_once($_SERVER['DOCUMENT_ROOT']."/IniConfig.php");
$access = "PUBLIC";
require_once(CONFPATH . "include_secure/checkgroup.php");

$path = array();
$file = array();

$path['templates'] = './mytemplates/';
$file['template'] = 'myindex.tpl.php';
$file['otherthing'] = 'otherthing.php';
$file['iamempty'] = '';

print ("path['templates'] = " . $path['templates'] . "&lt;br&gt;");
print ("file['template'] = " . $file['template'] . "&lt;br&gt;");
print ("file['otherthing'] = " . $file['otherthing'] . "&lt;br&gt;");
print ("file['iamempty'] = " . $file['iamempty'] . "&lt;br&gt;");

print ("file['file'] = " . $file['file'] . "&lt;br&gt;");
print ("file = " . $file);

//the Output is:
/*
path['templates'] = ./mytemplates/
file['template'] = myindex.tpl.php
file['otherthing'] = otherthing.php
file['iamempty'] =
file['file'] =
file = Array
*/
</code></pre>

That's how I realised how important it is to initialise variables as we never know what problem we might end up with later, and just for wanting to save time we might end up wasting even more at the end.
I hope this will be helpful for those like me who are not professional.

blocks|key|506469|text|老问题，但我分享这是因为这是一个用例，如果不声明数组，代码会更简单。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|506470|假设你有一个对象列表，你需要在它们的一个属性上建立索引。|506471|//+$list+is+array+of+objects,+all+having+$key+property.
$index+=+[];
foreach+($list+as+$item)
++++$index[$item->key][]+=+$item;+//+Dont+care+if+$index[$item->key]+already+exists+or+not.|code-block|syntax|javascript|506472|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Old question but I share this as this is one use case where code is simpler if arrays are not declared.

Say you have a list of objects you need to index on one of their properties.

<pre><code>// $list is array of objects, all having $key property.
$index = [];
foreach ($list as $item)
 $index[$item-&gt;key][] = $item; // Dont care if $index[$item-&gt;key] already exists or not.
</code></pre>

blocks|key|722163|text|这取决于您的错误检查。如果你在strict上有错误报告，它会给你一个通知，但从技术上讲，它应该仍然可以在没有它的情况下工作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|722164|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

It depends on your error checking. If you have error reporting on strict it'll give you a notice but it should still technically work without it.

blocks|key|506119|text|当您需要它作为全局变量或希望在不同函数中一次又一次地重用同一数组时，它很好用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|506120|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Its good in case when you need it as a global variable or want to reuse the same array again and again in different functions

blocks|key|722352|text|同意@djdy，只是我想要发布的一个替代方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|722353|<?php

//+Passed+array+is+empty,+so+we'll+never+have+$items+variable+available.
foreach+(array()+AS+$item)
++++$items[]+=+$item;

isset($items)+OR+$items+=+array();+//+Declare+$items+variable+if+it+doesn't+exist

?>|code-block|syntax|javascript|722354|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Agree with @djdy, just one alternative I'd love to post:

<pre><code>&lt;?php

// Passed array is empty, so we'll never have $items variable available.
foreach (array() AS $item)
 $items[] = $item;

isset($items) OR $items = array(); // Declare $items variable if it doesn't exist

?&gt;
</code></pre>

<pre><code>$arr = array(); // is this line needed?
$arr[] = 5;
</code></pre>

I know it works without the first line, but it's often included in practice. 

What is the reason? Is it unsafe without it?

I know you can also do this:

<pre><code> $arr = array(5);
</code></pre>

but I'm talking about cases where you need to add items one by one.

Is it necessary to declare PHP array before adding values with []?

$arr = array(); // is this line needed?$arr[] = 5;我知道它不需要第一行就可以工作，但在实践中它经常被包括在内。原因何在？没有它是不安全的吗？我知道你也可以这样做： $arr = array(5);但我说的是需要一个接一个地添加项目的情况。

问在用[]添加值之前，有必要声明PHP数组吗？
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问在用[]添加值之前，有必要声明PHP数组吗？EN

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问在用[]添加值之前，有必要声明PHP数组吗？
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