PHP Programming: Exploring the mysteries of strings _php

One, the quotation mark defines the string

In PHP, typically a string is defined in a pair of quotation marks, such as:

' I am a string in single quotes '
"I am a string in double quotes"

The PHP parser uses pairs of quotes to determine a string. Therefore, all strings must use the same single or double quotation mark to define the start and end. For example, the following string definitions are not valid:

"I am not a valid string since I had unmatching quote marks '
' Me neither! '

When defining a string, only one quotation mark is considered a definition, that is, single or double quotation marks. Then, if a string starts with double quotes, only the double quotes are parsed by the parser. This way, you can include any other characters, even single quotes, in a double-quote string. The following string of quotes is valid:

$s = "I am a ' single quote string ' inside a double quote string";
$s = ' I am a ' double quote string ' inside a single quote string ';

When PHP encounters the quotation marks corresponding to the beginning of the string, it is considered to be at the end of the strings, so:

"Why doesn ' t" this "work?"

is actually divided into three parts by the PHP parser:

"Why doesn ' t"--a double quote string containing a single quotation mark
this--extra characters, parser cannot process
"Work?"--ordinary string

The above example attempts to include double quotation marks in a double-quote string, and the parser considers the string to end when it encounters the second double quotation mark. To achieve the purpose of including quotation marks, the parser must ignore the literal quotes in the string when it is intended, we put a backslash in front of the quotation marks to tell PHP: This quotation mark is part of the string, the correct way to represent:

"Why doesn ' t \" that\ "work?"

A common problem in English strings is the use of apostrophes because it is a single quotation mark, which is very common in English strings. You must handle these characters with care:

' You\ ' d better escape your apostrophes '

You can see that the backslash has his special meaning in the string, and when we need to include the backslash itself in the string, we need to add a backslash before the symbol. For example:

$file = "C:\Windows\System.ini";
Echo $file; Printing Result: C:windowssystem.ini
$file = "C:\\windows\\system.ini";
Echo $file; Printing Result: C:\Windows\System.ini

Another way to define a string is to eliminate the annoyance of special characters and to make it easier to refer to longer text. The string defines the method to << <符号紧跟一个自定义字符串开头，最后一行以该自定义字符串结束，并且必须顶格。
<符号紧跟一个自定义字符串开头，最后一行以该自定义字符串结束，并且必须顶格。>


Second, the connection of strings

Strings can use string connectors (.) To connect, such as:

$first _name = ' Charlie ';
$last _name = ' Brown ';
$full _name = $first _name. ' ' . $last _name;

A common use is to create chunks of HTML string code, and the assignment number (=) connector (.) can be abbreviated to (. =) symbols, such as:

$html = '





'; $html. = ' '; for ($i =0; $i <10; $i + +) {$square = $i * $i; $html. = ' '; } $html. = '

Number	Square
' . $i. '	' . $square. '

';


Iii. using variables in strings

This feature allows you to glue and use a large number of simple strings without using connection symbols. PHP allows us to include string variables directly in a double quote string, and we can see that the following two strings are processed in the same way.

$full _name = $first _name. ' ' . $last _name;
$full _name = "$first _name $last _name";

The processing of single and double quotation marks in PHP is not the same. The contents of a double-quote string can be interpreted and replaced, and the contents of a single quote string are always considered ordinary characters. For example:

$foo = 2;
echo "Foo is $foo"; Print Result: Foo is 2
Echo ' foo is $foo '; Print Result: Foo is $foo
echo "Foo is $foo \ n"; Print Result: Foo is 2 (simultaneous line wrapping)
Echo ' foo is $foo \ n '; Print Result: Foo is $foo \ n

As you can see, even the backslash in the single quote string loses his extended meaning (except for inserting backslashes \ \ and inserting single quotation marks \ '). Therefore, you should use double quotation marks when you want to perform variable substitution in a string and include escape sequences such as \ n (line break). Single quote string can be used anywhere else, the script using single-quote string processing speed is faster, because the PHP parser on the single-quote string processing method is relatively simple, and double-quote processing because the string inside also need to parse, so more complex, so processing speed slightly slower.

When you reference a complex combination of variables in a string, some problems may arise, and the following code will work correctly:

echo "value = $foo";
echo "value = $a [$i]";

And the following code does not get the result we want:

echo "value = $a [$i] [$j]"; We want to print an element of a two-dimensional array of $ A.

To avoid the potential problems in the use of these strings, we often separate complex variables from strings, like this:

echo ' value = '. $a [$i] [$j];

Another option is to enclose the complex variable in curly braces, which the parser can correctly identify:

echo "value = {$a [$i] [$j]}"//prints an element of two-dimensional array $ A

In this way, new problems have arisen. When we want to refer to the curly brace character itself in a string, we'll remember to use the escape character:

$var = 3;
echo "value = {$var}"; Print result "value = 3"
echo "value = \{$var}"; Print Result "value = {3}"


Three, Slash, and SQL statements

Generating HTML code or SQL query statements is a common and interesting thing to do when writing PHP programs. Why do you say that, because this involves generating another type of code, you must carefully consider and follow the writing syntax and rules required by this code.

Let's look at an example where you want to query the database for users whose names are "O ' Keefe", usually in the form of SQL statements:

SELECT * from users where last_name = ' o\ ' Keefe '

Note that the SQL statement, the English-language (apostrophe), must be escaped with a backslash. PHP specifically provides functions to handle situations where the purpose of the function addslashes ($STR) is to automatically insert a backslash escape character in a string with a quote character:

$last _name = "O ' Keefe";
$sql = "SELECT * from users where last_name = '". Addslashes ($last _name). "'";

In this case, you'll also enclose the single quotation mark (SQL syntax requirement) outside the last_name string, so you don't need to use this pair of single quotes because you're using a double quote string. The following statement is the equivalent of using a single quote string:

$sql = ' SELECT * from users where last_name = \ '. Addslashes ($last _name). '\';

Any time you want to write a string in the database, you have to make sure that the quotes inside are correctly using the escape symbol, which is a common mistake for many PHP beginners.


Four, double quotes and HTML

Unlike SQL statements, double quotes are often used to denote strings in the standard HTML language (many browsers now have strong fault tolerance, allowing single quotes or even no quotes in HTML), for example:

$html = ". $link.";
$html = "$link";

The HTML language does not support backslash escaping, which is something that we experience when we use the form's hidden inputs to transfer data. The best way to set the value of hidden inputs is to encode it using the Htmlspecialchars () function. The following statement can normally transmit a data that may contain double quotes:

　　 