注意：PHP没有对string的长度做限制。唯一限制的就是PHP在计算机中的可用内存（php.ini文件中的memory_limit变量的值)
限定字符串范围的方法有4中：
1、单引号；
2、双引号；
3、原型文档语法；
4、nowdoc syntax（PHP5.3.0开始）
1、如果字符串使用单引号“‘”包裹，字符串中如果出现单引号“，”和反斜杠“”符号，需要进行转义。
复制代码 代码如下:
// Outputs: Arnold once said: "I'll be back"
echo 'Arnold once said: "I'll be back"';
// Outputs: You deleted C:*.*?
echo 'You deleted C:*.*?';
// Outputs: You deleted C:*.*?
echo 'You deleted C:*.*?';
（有待验证 单引号包裹的字符串反斜杠是否需要转义）
2、如果字符串被双引号包裹 一下字符都会被转义：
Escaped characters Sequence Meaning
n linefeed (LF or 0x0A (10) in ASCII)
r carriage return (CR or 0x0D (13) in ASCII)
t horizontal tab (HT or 0x09 (9) in ASCII)
v vertical tab (VT or 0x0B (11) in ASCII) (since PHP 5.2.5)
f form feed (FF or 0x0C (12) in ASCII) (since PHP 5.2.5)
backslash
$ dollar sign
" double-quote
[0-7]{1,3} the sequence of characters matching the regular expression is a character in octal notation
x[0-9A-Fa-f]{1,2} the sequence of characters matching the regular expression is a character in hexadecimal notation
如果字符串 使用双引号“"”或者原形文档语法的形式包裹的话，在字符串中的变量会被解析。
1、简单语法：
因为解析器会贪婪匹配$后面的字符，所以，为了不出什么以外，应该使用"{"和"}"来表名变量的边界。
复制代码 代码如下:
<?php
$beer = 'Heineken';
echo "$beer's taste is great"; // works; "'" is an invalid character for variable names
echo "He drank some $beers"; // won't work; 's' is a valid character for variable names but the variable is "$beer"
echo "He drank some ${beer}s"; // works
echo "He drank some {$beer}s"; // works
?>
同样，数组的下标和对象的属性也会不解析。
复制代码 代码如下:
<?php
// These examples are specific to using arrays inside of strings.
// When outside of a string, always quote array string keys and do not use
// {braces}.
// Show all errors
error_reporting(E_ALL);
$fruits = array('strawberry' => 'red', 'banana' => 'yellow');
// Works, but note that this works differently outside a string
echo "A banana is $fruits[banana].";
// Works
echo "A banana is {$fruits['banana']}.";
// Works, but PHP looks for a constant named banana first, as described below.
echo "A banana is {$fruits[banana]}.";
// Won't work, use braces. This results in a parse error.
echo "A banana is $fruits['banana'].";
// Works
echo "A banana is " . $fruits['banana'] . ".";
// Works
echo "This square is $square->width meters broad.";
// Won't work. For a solution, see the complex syntax.
echo "This square is $square->width00 centimeters broad.";
?>
2、复合语法:
复制代码 代码如下:
<?php
// Show all errors
error_reporting(E_ALL);
$great = 'fantastic';
// Won't work, outputs: This is { fantastic}
echo "This is { $great}";
// Works, outputs: This is fantastic
echo "This is {$great}";
echo "This is ${great}";
// Works
echo "This square is {$square->width}00 centimeters broad.";
// Works
echo "This works: {$arr[4][3]}";
// This is wrong for the same reason as $foo[bar] is wrong outside a string.
// In other words, it will still work, but only because PHP first looks for a
// constant named foo; an error of level E_NOTICE (undefined constant) will be
// thrown.
echo "This is wrong: {$arr[foo][3]}";
// Works. When using multi-dimensional arrays, always use braces around arrays
// when inside of strings
echo "This works: {$arr['foo'][3]}";
// Works.
echo "This works: " . $arr['foo'][3];
echo "This works too: {$obj->values[3]->name}";
echo "This is the value of the var named $name: {${$name}}";
echo "This is the value of the var named by the return value of getName(): {${getName()}}";
echo "This is the value of the var named by the return value of $object->getName(): {${$object->getName()}}";
访问,修改字符串中的指定字符：
字符串可以使用"[]"和"{}"进行访问。（注意：php5.3.0以后不建议使用“｛｝”访问）
注意：使用其他类型（非integer）类型访问字符串指定的字符，都会返回NULL
警告：
Writing to an out of range offset pads the string with spaces. Non-integer types are converted to integer. Illegal offset type emits E_NOTICE. Negative offset emits E_NOTICE in write but reads empty string. Only the first character of an assigned string is used. Assigning empty string assigns NUL byte。