Single quotes and double quotes in 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 marks 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. So if a string is represented by a double-cited
number, then only double quotes are parsed by the parser. in this way, you can include any other character in a double-quote string, or even a single-cited
No. The following string of quotes is valid:

$s = "I'll pass MCSE 2003 certifications ";
$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
This example attempts to include double quotation marks in a double-quote string, and the parser considers the string knot when it encounters the second double quotation mark.
A bundle. To achieve the purpose of containing quotation marks, the parser must ignore the literal quotes in the string, and we
Preceded by a backslash to tell PHP: This quotation mark is part of the string, the correct way to represent it:

"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.
(All in English). 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
The symbol is preceded by a backslash. 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 party
The <<< symbol immediately begins with a custom string, the last line ends with the custom string, and must be shelf.

Second, the connection of strings

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

$first _name = 'MCSE 2003 bootcamp';
$last _name = ' MCP bootcamp';
$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 (. =) characters
Numbers, such as:

$html = ' <table> ';
$html. = ' <tr><td>number</td><td>square</td></tr> ';
for ($i =0; $i <10; $i + +) {
$square = $i * $i;
$html. = ' <tr><td> '. $i. ' </td><td> '. $square. ' </td></tr> ';
}
$html. = ' </table> ';

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 words directly in a double quote string
String variable, we can find that the following two string processing results are the same.

$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 the double-quote string can be interpreted and replaced, and the single-citation
The contents of the string are always considered to be 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, in a single quote string even the backslash also loses his extended meaning (in addition to inserting a backslash \ \ and inserting a single
quotation marks \ '). So, when you want to do variable substitution in strings and escape sequences that contain \ n (newline characters), you should use double-cited
No. Single-quote strings can be used anywhere else, and it is faster to use single-quote strings in scripts, because the PHP parser
Single-Quote string processing method is relatively simple, and double-quote processing because the string internal also need to parse, so more complex, so the 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}"

Four, 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 syntax and rules required by this Code
The

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
That is true:

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 some functions to handle such
, the purpose of the function addslashes ($STR) is to automatically insert a backslash escape character in a string with the quote character:

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

In this example, you will also enclose the single quotation mark (SQL syntax requirement) outside the last_name string, as this is used for dual
Quoted string, so no escaping is necessary for this pair of single quotes. 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 lot of PHP
Mistakes that beginners often make.

V. Double quotes and HTML

Unlike SQL statements, double quotes are often used to denote strings in standard HTML languages (many browsers now have strong fault-tolerant
Can, allow single quotes in HTML or even without quotation marks to denote strings), for example:

$html = ' <a href= '. $url. ' " > '. $link. ' </a> ';
$html = "<a href=\" $url \ "> $link </a>";

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

<typenamevalue= "<? PHP Echo htmlspecialchars ($var)?> ">

Quotation marks define a string. To achieve the purpose of including quotation marks, the parser must ignore the literal quotes in the string, ignoring its original intent, 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: the single quote string can be used anywhere else, It is faster to use single-quote string processing in a script, because the PHP parser handles single-quote strings more simply, and the processing of double-quotes is more complicated because the strings need to be parsed internally, so the processing speed is slightly slower.

This one... Double quotes escaped, single quotes not escaped
such as:/r/n is a newline, but if you write to the file in single quotation marks, it is not a newline, but a character, if the file is written in double quotation marks, it is a newline.

Single quotes and double quotes in PHP