php 验证，如果字符串以给定的子字符串开头
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verify, if the string starts with given substring
提问by Simon
i have a string in $strvariable.
我有一个$str变量字符串。
how can i verify is it starts with some word?
我如何验证它是否以某个词开头？
example
例子
$str = "http://somesite.com/somefolder/somefile.php";
when i wrote the following script returns yes
当我编写以下脚本时返回 yes
if(strpos($str, "http://") == '0') echo "yes";
BUT it returns yeseven when i wrote
但是即使我写了它也会返回yes
if(strpos($str, "other word here") == '0') echo "yes";
i think it strposreturns zeroif it can't find substring too(or empty value).
我认为如果它也找不到子字符串（或空值），它就会strpos返回zero。
so, what can i do, if i want to verify the word, which in the start of string?(maybe i must use ===in this case?)
那么，如果我想验证字符串开头的单词，我该怎么办？（也许===在这种情况下我必须使用？）
回答by cletus
You need to do:
你需要做：
if (strpos($str, "http://") === 0) echo "yes"
The ===operator is a strict comparison that doesn't coerce types. If you use ==then false, an empty string, null, 0, an empty array and a few other things will be equivalent.
该===运营商进行严格比较并不强制类型。如果使用==then false，则空字符串、null、0、空数组和其他一些东西将是等效的。
See Type Juggling.
请参阅类型杂耍。
回答by Dennis Haarbrink
You should check with the identity operator(===), see the documentation.
Your test becomes:
你的测试变成：
if(strpos($str, "http://") === 0) echo "yes"; //returns yes
回答by Ray
PHP does have 2 functions to verify if a string starts with a given substring:
PHP 确实有 2 个函数来验证字符串是否以给定的子字符串开头：
strncmp(case sensitive);strncasecmp(case insensitive);
strncmp（区分大小写）;strncasecmp（不区分大小写）;
So if you want to test only http (and not https), you can use:
因此，如果您只想测试 http（而不是 https），则可以使用：
if (strncasecmp($str,'http://',7) == 0) echo "we have a winner"
回答by Raja
check with
检查
if(strpos($str, "http://") === 0) echo "yes";
as ==will turn positive for both false & 0 check the documentation
如==将转均为阳性假0检查文档
回答by SimonW
Another option is:
另一种选择是：
if (preg_match("|^(https?:)?\/\/|i", $str)) {
echo "the url starts with http or https upper or lower case or just //.";
}
As shown here: http://net.tutsplus.com/tutorials/other/8-regular-expressions-you-should-know/
如图所示：http: //net.tutsplus.com/tutorials/other/8-regular-expressions-you-should-know/
回答by Luca C.
strncmp($str, $word, strlen($word))===0
Is a bit more performant than strpos
性能比 strpos 高一点
回答by Ben Everard
if(substr($str, 0, 7)=="http://") {
echo("Statrs with http://");
}
回答by deceze
There's a big red warning in the documentationabout this:
文档中有一个很大的红色警告：
This function may return Boolean FALSE, but may also return a non-Boolean value which evaluates to FALSE, such as 0 or "". Please read the section on Booleans for more information. Use the === operator for testing the return value of this function.
此函数可能返回布尔值 FALSE，但也可能返回计算结果为 FALSE 的非布尔值，例如 0 或“”。请阅读有关布尔值的部分以获取更多信息。使用 === 运算符来测试此函数的返回值。
strposmay return 0or false. 0is equal to false(0 == false). It is not identicalto false however, which you can test with 0 === false. So the correct test is if (strpos(...) === 0).
strpos可能返回0或false。0等于false（0 == false）。这是不相同的，以假然而，你可以测试0 === false。所以正确的测试是if (strpos(...) === 0).
Be sure to read up on the difference, it's important: http://php.net/manual/en/language.operators.comparison.php
请务必阅读差异，这很重要：http: //php.net/manual/en/language.operators.comparison.php
