Example analysis of type judgment in PHP weakly typed language

This article mainly introduces the type judgment operation in PHP weakly typed language, and analyzes the conversion, judgment and function usage skills and precautions for data types in PHP with examples. Friends in need can refer to it

The example in this article describes the type judgment operation in PHP weakly typed language. Share it with everyone for your reference, the details are as follows:

1. When PHP compares a number with a string or performs operations, PHP will convert the string into a number and then compare it. The rules of PHP conversion are: if the string starts with a number, the starting number will be used as the conversion result, if not, 0 will be output.

For example: 123abc should be 123 after conversion, and abc is 0, 0==0. This is of course true. For details, please refer to the official manual: If you compare an integer and a string, the character The string will be converted into an integer

$a = ($b=4)+5;
echo $a;  //9
echo &#39;<p>&#39;;
var_dump(1==&#39;1a&#39;);  //true
echo &#39;<p>&#39;;
var_dump(1==&#39;2a&#39;);  //false
echo &#39;<p>&#39;;
var_dump(1==&#39;a1&#39;);  //false
echo &#39;<p>&#39;;
var_dump("1"==&#39;1a&#39;);  //false
echo &#39;<p>&#39;;
var_dump(&#39;51a&#39;+6);  //57
echo &#39;<p>&#39;;
var_dump(&#39;a51&#39;+6);  //6

2. In PHP, == will perform type conversion first and then compare, while === will compare first Type, if the types are different, unequal will be returned directly. Refer to the following example

$a = null;
$b = '';
$c = 0;
echo ($a == $b) ? 1 : 0;  // 输出1
echo ($a === $b) ? 1 : 0;  // 输出0
echo ($a == $c) ? 1 : 0;  // 输出1
echo ($a === $c) ? 1 : 0;  // 输出0
echo ($b == $c) ? 1 : 0;  // 输出1
echo ($b === $c) ? 1 : 0;  // 输出0

3. Empty and isset functions in PHP

1) When the variable is: 0, "0", null, '', false, array(), use the empty function, and the returned value is true

2) When the variable is undefined or null , the isset function returns false, and the others are not true

$a = null;
$b = '';
$c = 0;
$d = "0";
$e = false;
$f = array();
var_dump(empty($a));  //true
echo &#39;<p>&#39;;
var_dump(empty($b));  //true
echo &#39;<p>&#39;;
var_dump(empty($c));  //true
echo &#39;<p>&#39;;
var_dump(empty($d));  //true
echo &#39;<p>&#39;;
var_dump(empty($e));  //true
echo &#39;<p>&#39;;
var_dump(empty($f));  //true
echo &#39;<p>&#39;;
var_dump(isset($a));  //false
echo &#39;<p>&#39;;
var_dump(isset($b));  //true
echo &#39;<p>&#39;;
var_dump(isset($c));  //true
echo &#39;<p>&#39;;
var_dump(isset($d));  //true
echo &#39;<p>&#39;;
var_dump(isset($e));  //true
echo &#39;<p>&#39;;
var_dump(isset($f));  //true
echo &#39;<p>&#39;;
var_dump(isset($g));  //false

4. When using functions such as strpos, you must use equality to judge

$sms = "abc";
if(strpos($sms,'a') != false ){
  echo 1;
}else{
  echo 2;
}

In this case, it is obviously to determine whether the abc string contains a. The expected result should be 1, and the actual result is 2. This is because when the strpos function cannot match the target string, it returns false. When it matches the target string, it returns the target string. At the index position in the search string, 0 is returned here.

$sms = "abc";
if(strpos($sms,'a') !== false ){
  echo 1;
}else{
  echo 2;
}

It is correct to change it to constant equal.

5. Double quotation marks are required for line breaks

##

$time = date('Y-m-d H:i:s').'\r\n';
file_put_contents('filename.txt', $time , FILE_APPEND);

If written like this, the file will look like this when opening it 2016-09-02 08: 04:04\r\n2016-09-02 08:04:05\r\n2016-09-02 08:04:05\r\n2016-09-02 08:04:05\r\n2016-09-02 08:04:22

The correct way to write it is\r\n, use double quotes

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