Because PHP is a weakly typed language, sometimes unexpected results will appear when comparing. Today we will summarize some situations of PHP variable comparison.
First off, only values of the same data type can be compared, and PHP is no exception. It's just that PHP converts values of different types to the same data type before comparing them. In this way, the value comparison problem in PHP is converted into a conversion problem between different data types in PHP.
String and String Comparison
PHP string comparison is generally a bitwise comparison, so the following expressions can get the results you want.
var_dump ('' == 'null'); // false var_dump ('' == '0'); // false
However, once the content of the string is a legal numerical expression (including scientific notation), even if the comparison is all strings, PHP will perform data type conversion, first convert the string to the corresponding integer or float and then compare.
var_dump('0' == '00'); //true var_dump('100' == '1E2'); //true var_dump('.12' == '0.12'); //true var_dump('12aaa ' == '12'); //false because '12aaa' is not all numbers, so no type conversion is performed, and bitwise comparison is still performed var_dump('12aaa' == 12); //true because the right side of the comparison operator is an integer type 12, so first perform type conversion, then compare
What if we == replace it with the ===result? Let's take a look
var_dump ('0' === '00'); // false var_dump ('100' === '1E2'); // false var_dump ('. 12' === '0.12'); // false var_dump ('12aaa' === '12'); // false var_dump ('12aaa' === 12); // false
It can be seen that the full equal sign ===will strictly perform bitwise comparison for PHP string comparisons, and will not convert even strings of numeric expressions.
String and boolean comparison
If the types are different, such as comparing strings and booleans, PHP first converts the strings to booleans and then compares them. Let's first look at a set of Demos
var_dump ('' == true); // false var_dump ('0' == true); // false var_dump ('1' == true); // true var_dump ('00 '== true); // true var_dump ('0q' == true); // true var_dump ('a0' == true); // true var_dump ('null' == true); //threaten
The rules for converting PHP strings to booleans are
false if the string is empty; true if the string is not empty; '0' is false
Value comparison of string and number types
If a string is compared with a value of data type, first the string is converted to a value of type number, and then the comparison is performed. The conversion rules are as follows:
If the string is a legal numeric expression (including scientific notation), it is converted to the corresponding number; if the string begins with a digit, or begins with a decimal point followed by a digit, the leading part of the numeric expression is taken converts it to a number; if the string starts with a non-digit, or if the decimal point is followed by a non-digit, it is converted to 0.
So there will be the following result
var_dump ('adkf' == 0); // true var_dump ('0asda' == 0); // true var_dump ('1ad' == 0); // false var_dump ('adkf' == '0asda'); // false var_dump ('adkf' == '0'); // false var_dump ('0asda' == '0'); // false var_dump ('. 12ads' == 0.12); // true var_dump ('E2ada' == 100); // false var_dump ('1E2asd' == 100); // true var_dump ('1E2asd' == '100'); // false
about null
Undefined variables and properties in PHP are equal to Null, so there will be the following results
$a = null;$b;var_dump(isset($a)); //false var_dump(isset($b)); //false var_dump(isset($c)); //false class A{ public $v1 = null; public $v2; } $o = new A(); var_dump(isset($o->v1)); //false var_dump(isset($o->v2)); //false var_dump(isset( $o->v3)); //false var_dump($c == null); //There will be Notice error, but the result is true var_dump($c === null); //There will be Notice error, but the result is true var_dump(is_null($c)); //There will be Notice error, but the result is true
other
There are also some following examples, which also involve type conversion
var_dump ([] == false); // true var_dump ('' == false); // true var_dump (null == false); // true var_dump ('' == null); // true var_dump ([] == null); // true var_dump ('' == []); // false
Summarize
Due to the weak type feature of PHP, unexpected results sometimes occur when comparing or if judgments are made. So when making judgments in PHP, be sure to keep in mind the types of different variables. In order to prevent unexpected problems, you can use all equals ===or string comparison functions strcmpto judge
Note: The above code all use the 7.3 version to get the result