How to use php comparison operators? Detailed explanation of comparison operator usage

Comparison OperatorsAllows comparison of two values. The comparison operators are as follows:

1) $a > $b is greater than: if $a is strictly greater than $b, then TRUE is returned
2) $a < $b is less than: if $ If a is strictly less than $b, then TRUE
3) $a >= $b is greater than or equal to: If $a is greater than or equal to $b, then TRUE
4) $a <= $b is less than or equal to: If $a is less than or equal to $b, return TRUE
5) $a <> $b is not equal to: If $a is not equal to $b, return TRUE
6) $a != $b No Equal to: If $a is not equal to $b, return TRUE (same as above)
7) $a == $b Equal to: If $a is equal to $b, return TRUE
8) $a === $ b is equal to: if $a is equal to $b, and their types are also the same, then return TRUE
9) $a !== $b is not equal to: if $a is not equal to $b, or their types are different, then returns TRUE

Among them, we need to focus on distinguishing between "equal to" and "equal to". $a == $b only compares the values ​​​​of the two variables, while equal to the operator The expressions on both sides perform value comparison and data type comparison at the same time. Only when the values ​​on both sides are equal, the operation result is "true". Combined with the "inequality" operator, for example, $a = 2; var_dump($a!==2); this expression returns "false" because 2 is equal to 2. In addition, $a = 2; is an integer type and 2 in var_dump($a!==2); is also an integer type, but the operator is not exactly equal to "!==", so the result is false, because 2 is equal to 2 . On the other hand, if this is the case $a = 2; var_dump($a!=='2'); the result of the operation is "true", because 2 is not equal to '2', and the following '2' is a string of '2' ', that is, not all equals not only compare the variable values, but also compare the data types of the variables.

Comparing results of different types in PHP

If the PHP comparison operator compares an integer and a string, the string will be converted to an integer and compared. If you compare two numeric strings, they are compared as integers, and this rule also applies to switch statements.
For example:
var_dump(0 == "a"); // Returns TRUE, "a" is converted to 0
var_dump("1" == "01"); // Returns TRUE, Treat as an integer

Comparison between string or null and string: Convert NULL to "", compare numbers or strings
Comparison between bool or null: Convert to bool, FALSE < TRUE
Object built-in classes can define their own comparisons. Different classes cannot be compared, and properties of the same class are compared
Comparison between strings, resources or numbers: Convert strings and resources into numbers, and compare them according to ordinary mathematics
array Comparison between: Arrays with fewer members are smaller. If the key in operand 1 does not exist in operand 2, the arrays cannot be compared and need to be compared value by value (see the following code)
array and any other type Comparison: array is always larger
Comparison of object and any other type: object is always larger

1. For array

$a=array(1,2,3,6); 
$b=array(1,2,3,6,8); 
echo "\n"; 
var_dump( $a>$b); 
var_dump( $a==$b); 
var_dump( $a<$b);

Result:

boolean false 
boolean false 
boolean true

ps: Arrays with fewer members are smaller

$a=array(1,2,3,6,9); 
$b=array(1,2,3,6,8); 
echo "\n"; 
var_dump( $a>$b); 
var_dump( $a==$b); 
var_dump( $a<$b); 
boolean true 
boolean false 
boolean false

ps: Value-by-value comparison

$a=array(1,2,3,6,'b'=>3); 
$b=array(1,2,3,6,8); 
echo "\n"; 
var_dump( $a>$b); 
var_dump( $a==$b); 
var_dump( $a<$b); 
boolean false 
boolean false 
boolean false

ps: If the key in operand 1 does not exist In operand 2, the arrays cannot be compared, and false is returned.

Array comparison code:

function standard_array_compare($op1, $op2)
{
  if (count($op1) < count($op2)) {    //具有较少成员的数组较小
    return -1;    // $op1 < $op2
  } elseif (count($op1) > count($op2)) {
    return 1;    // $op1 > $op2
   } 
 
  foreach ($op1 as $key => $val) {
   if (!array_key_exists($key, $op2)) {
      return null;   
   } else if ($val < $op2[$key]) {
      return -1;
    } elseif ($val > $op2[$key]) {
      return 1;
     }
  }
  return 0;   // $op1 == $op2
}

2. Comparison between bool or null and other types

var_dump((bool)(null));//boolean false //When null is converted to bool, it is false;

null and other types are converted to bool, and then compared, and FALSE < TRUE

Comparison operation The ternary operation symbol in the symbol:

Expression (expr1) ? (expr2) : (expr3) When the value of expression expr1 is TRUE, the value is expr2. When the expression The value of expr1 is expr3 when it is FALSE.

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