php function eval () Introduction and use example, eval example _php tutorial

php function eval () Introduction and use example, eval example

What is eval ()?

The eval () function calculates the string according to the PHP code.
The string must be a valid PHP code and must end with a semicolon.
If no return statement is called in the code string, NULL is returned. If there is a parsing error in the code, the eval () function returns FALSE.

Grammar

Eval (Phpcode)
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Parameter description
Phpcode required. Specifies the PHP code to be computed.

Example 1
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<?php
$string = "Beautiful";
$time = "Winter";

$str = ' This is a $string $time morning! ';
echo $str. "
";

Eval ("\ $str = \" $str \ ";");
Echo $str;
?>

Output:
This is a $string $time morning!
This is a beautiful winter morning!

Example 2

We use a For loop to create n random, and the value multiplies
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<?php

for ($i =1; $i <=10; $i + +) {
Eval (' $a '. $i. ' = '. ( $i * $i). '; ');
}

for ($i =1; $i <=10; $i + +) {
Eval (' echo $a ' $i. '. \ '
\' ;');
}

Echo '
';
Echo $a 1 + $a 10;

Output:
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1
4
9
16
25
36
49
64
81
100
101

How to use the eval () function in PHP

Eval () must be a string in parentheses

If you write $a = 1; Eval ("Echo (\ $a +\ $b);"); Operation result is 1

If it is $a = 1; Eval ("Echo ($a + $b);"); The error

This means it's best to have a strict string and a full PHP statement.

$a = 1; eval ("echo \ $a;");
$a = 1; Eval ("echo $a;");
As a result, the first eval was performed with the Echo $a; And the second sentence is to perform echo 1;

How does PHP use the eval () function to change this instance?

$f = ' $ ';

$f. $a. $this->i
What this is, obviously, should not be the effect you want, which is equivalent to a string connection. Presumably you mean to do this, $ ($a. $this->i). Let's just think about the rest.

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