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Detailed explanation of PHP syntax rules for introductory learning_PHP tutorial
Detailed explanation of PHP syntax rules for introductory learning_PHP tutorial
1. Simple syntax rules (delimit variable names with curly braces, applicable to all versions of PHP):
$a ='flower';
echo "She received some $as";// Invalid; the letter s will be regarded as a valid variable name to form an element, but the variable here is $a
echo "She received some ${a}s";// Valid
echo "She received some {$a}s";// Valid; recommended usage
What we want to express is" "She received some flowers", the word "flower" in the context should be in the plural form (that is, S should be added after it), but if the variable is not defined in any way, the first echo situation will occur. Obviously we want the output to be $a rather than $as. So how do we usually process this output?
echo "She received some $a"."s";
echo " She received some ".$a."s";// These two customary ways of writing should be less concise and clear than the way of writing with curly brackets, right?
Note: No matter whether { appears before or after $, curly braces will be regarded as delimiting symbols only when they are immediately adjacent. Do not add spaces between them, otherwise they will be treated as ordinary curly braces
echo "She received some { $a}s";// The output result is: She received some { flower} s
2. Complex syntax rules (delimit expressions with braces, etc., used with PHP4+):
echo "Valid writing method: {$arr[4][3]}";// Valid; define multi-dimensional array
echo "Valid writing method: {$arr[' foo'][3]}"; // Valid; when using a multi-dimensional array in a string, be sure to enclose it in parentheses
echo "Valid writing: {$this->width}00" ;// Valid; if not defined, it will become $this->width00
echo "Effective writing: {$this->value[3]->name}";// Valid; This example demonstrates the valid writing method of defining the chain call
echo ": $name: {${$name}}";// is valid; the effect demonstrated by this example is actually a variable variable
echo " Valid writing method: {${getName()}}";// Valid; this example demonstrates how to use the return value of the function as a variable name
echo " to effectively issue: {${$this->getName( )}}";// Valid; this example demonstrates using the return value of the function as a variable name
Note 1: echo "Is it valid to write like this: {getName()}"; the output result is :'Is it valid to write like this:
{getName()}'. Because there is no $ in it, the curly braces will not be used as delimiters
Note 2: echo "Is it valid to write like this: {$arr[foo][3]}"; Let's do it first before answering this question An experiment:
error_reporting(E_ALL);
$arr = array('a ','b','c','d'=>'e');
echo "This is $arr[d]";// We find that there is no problem in writing like this, then we like the following How to write it like this?
echo $arr[d];
produced this error:
Notice: Use of undefined constant d - assumed 'd'
Note: Use of undefined The constant d should probably be 'd'
So if we modify the code as follows
error_reporting(E_ALL);
$arr = array('a','b','c','d'=>'e');
define('f',' d');
echo $arr[f];
We found that there is no problem this time. It can be seen that there is no problem if the index of the array in the string is not enclosed in single quotes, but if this writing method does not appear in the string, an error will be reported, and for {$arr[foo][3]} in the string Parsing is based on non-string parsing. Therefore, it is wrong to only add curly braces to delimit the array in the string without adding single quotes to the index. Because the program interprets unquoted indexes as constants, an error occurs. The correct way of writing should be:
echo "Effective way of writing: {$arr['foo'][3]}";
A special reminder: echo "This is $arr[d ]";Although this writing method can be parsed by the program, it is limited to the case where the array is a one-dimensional array. The rigorous writing method should be: echo "This is {$arr['d']}"; My student once argued with me on this point. He said: Since the previous writing method can produce results, why must we use it? What about the latter way of writing? Then, let’s continue to modify the previous code
error_reporting(E_ALL);
$arr = array('a','b','c','d'=>array('e'=>'f')) ;
echo "This is $arr[d][e]";
Can this still be parsed correctly? I just want to tell you that adding curly braces is strictly necessary. Of course, if you are not my student, then I can’t control that much...
Note 3:
error_reporting(E_ALL);
$arr = array('a','b','c','d');
echo "This is {$ arr[2]}
";
echo "This is {$arr['2']}
";
Execute the above code . The result is the same, why is this? I can only tell you that PHP is a weakly typed language. As for what a weakly typed language is, I won’t say more here. Go Google it yourself. Having said so much, where are the specific applications that best reflect the advantages of these syntactic rules? ----SQL statement
// Example 1:
$SQL1 =" select * from table where id={$_GET['id']}";// Example 2:
$SQL2 ="select * from table where id={$this->id}";
OK, we’re done playing with curly braces here.
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