if
(PHP 4, PHP 5, PHP 7)
if结构是很多语言包括 PHP 在内最重要的特性之一，它允许按照条件执行代码片段。PHP 的if结构和 C 语言相似：
<?php
if (expr)
statement
?>
如同在表达式一章中定义的，expr按照布尔求值。如果expr的值为TRUE，PHP 将执行statement，如果值为FALSE——将忽略statement。有关哪些值被视为FALSE的更多信息参见转换为布尔值一节。
如果$a大于$b，则以下例子将显示a is bigger than b：
<?php if ($a > $b) echo "a is bigger than b"; ?>
经常需要按照条件执行不止一条语句，当然并不需要给每条语句都加上一个if子句。可以将这些语句放入语句组中。例如，如果$a大于$b，以下代码将显示a is bigger than b并且将$a的值赋给$b：
<?php
if ($a > $b) {
echo "a is bigger than b";
$b = $a;
}
?>
if语句可以无限层地嵌套在其它if语句中，这给程序的不同部分的条件执行提供了充分的弹性。
easy way to execute conditional html / javascript / css / other language code with php if else: <?php if (condition): ?> html code to run if condition is true <?php else: ?> html code to run if condition is false <?php endif ?>
Any variables defined inside the if block will be available outside the block. Remember that the if doesn't have its own scope.
<?php
$bool = true;
if ($bool) {
$hi = 'Hello to all people!';
}
echo $hi;
?>
It will print 'Hello to all people!'
On the other hand, this will have no output:
<?php
if (false) {
$hi = 'Hello to all people!';
}
echo $hi;
?>
re: #80305 Again useful for newbies: if you need to compare a variable with a value, instead of doing <?php if ($foo == 3) bar(); ?> do <?php if (3 == $foo) bar(); ?> this way, if you forget a =, it will become <?php if (3 = $foo) bar(); ?> and PHP will report an error.
An other way for controls is the ternary operator (see Comparison Operators) that can be used as follows: <?php $v = 1; $r = (1 == $v) ? 'Yes' : 'No'; // $r is set to 'Yes' $r = (3 == $v) ? 'Yes' : 'No'; // $r is set to 'No' echo (1 == $v) ? 'Yes' : 'No'; // 'Yes' will be printed // and since PHP 5.3 $v = 'My Value'; $r = ($v) ?: 'No Value'; // $r is set to 'My Value' because $v is evaluated to TRUE $v = ''; echo ($v) ?: 'No Value'; // 'No Value' will be printed because $v is evaluated to FALSE ?> Parentheses can be left out in all examples above.
You can have 'nested' if statements withing a single if statement, using additional parenthesis.
For example, instead of having:
<?php
if( $a == 1 || $a == 2 ) {
if( $b == 3 || $b == 4 ) {
if( $c == 5 || $ d == 6 ) {
//Do something here.
}
}
}
?>
You could just simply do this:
<?php
if( ($a==1 || $a==2) && ($b==3 || $b==4) && ($c==5 || $c==6) ) {
//do that something here.
}
?>
Hope this helps!In addition to the traditional syntax for if (condition) action; I am fond of the ternary operator that does the same thing, but with fewer words and code to type: (condition ? action_if_true: action_if_false;) example (x > y? 'Passed the test' : 'Failed the test')
This is aimed at PHP beginners but many of us do this Ocasionally...
When writing an if statement that compares two values, remember not to use a single = statement.
eg:
<?php
if ($a = $b)
{
print("something");
}
?>
This will assign $a the value $b and output the statement.
To see if $a is exactly equal to $b (value not type) It should be:
<?php
if ($a == $b)
{
print("something");
}
?>
Simple stuff but it can cause havok deep in classes/functions etc...It seems that only numbers can be compared between them but actually an alphabet can be compare too. For example :
<?php
// Number comparison
$a="C";
$b="X";
if ($a<$b)
{
echo $a."is smaller than".$b;
}
// Result : C is smaller than X
?>
As an added note to the guy below, in such a case, use the !== operator like this.
$nkey = array_search($needle, $haystack);
if ($nkey !== false) { ...
The !== and the === compare the "types". So, with this type of comparision, 0 is not the same as the FALSE returned by the array_search array when it can not find a match. :)
Quoted Text:
===================================
Be careful with stuff like
if ($nkey = array_search($needle, $haystack)) { ...
if the returned key is actually the key 0, then the if won't be executed
===================================You can do IF with this pattern : <?php $var = TRUE; echo $var==TRUE ? 'TRUE' : 'FALSE'; // get TRUE echo $var==FALSE ? 'TRUE' : 'FALSE'; // get FALSE ?>
You can also check alphabet characters like this
<?php
// Alphabetical Comparison
$a="brian";
$b="zebra";
if ($a < $b){
echo $a." is before ".$b." in the alphabet";
}
else{
echo $a." is after ".$b." in the alphabet";
}
// Result : brian is before zebra in the alphabet
?>
When using if statements without the curly braces, remember than only one statement will be executed as part of that condition. If you want to place multiple statements you must use curly braces, and not just put them on the same line. <?php if (1==0) echo "Test 1."; echo "Test 2"; ?> Whereas some people would expect nothing to be displayed, this piece of code will show: "Test 2".
@henryk (and everybody): You should put your arguments in order by *least* likely to be true. That way if php is going to be able to quit checking, it will happen sooner rather than later, and your script will run (what amounts to unnoticeably) faster. At least, that makes the most sense to me, but I don't claim omniscience.
This has got the better part of my last 2 hours, so I'm putting it here, maybe it will save someone some time. I had a if (function1() && function2()) statement. Before returning true or false, function1() and function2() had to output some text. The trick is that, if function1() returns false, function2() is not called at all. It seems I should have known that, but it slipped my mind.
Note that safe type checking (using === and !== instead of == and !=) is in general somewhat faster. When you're using non-safe type checking and a conversion is really needed for checking, safe type checking is considerably faster.
===================================
Test (100,000,000 runs):
<?php
$start = microtime(true);
for($i = 0; $i < 100000000; $i++)
if(5 == 10) {}
$end = microtime(true);
echo "1: ".($end - $start)."<br />\n";
unset($start, $end);
$start = microtime(true);
for($i = 0; $i < 100000000; $i++)
if('foobar' == 10) {}
$end = microtime(true);
echo "2: ".($end - $start)."<br />\n";
unset($start, $end);
$start = microtime(true);
for($i = 0; $i < 100000000; $i++)
if(5 === 10) {}
$end = microtime(true);
echo "3: ".($end - $start)."<br />\n";
unset($start, $end);
$start = microtime(true);
for($i = 0; $i < 100000000; $i++)
if('foobar' === 10) {}
$end = microtime(true);
echo "4: ".($end - $start)."<br />\n";
unset($start, $end);
?>
===================================
Result (depending on hardware configuration):
1: 16.779544115067
2: 21.305675029755
3: 16.345532178879
4: 15.991420030594Re : henryk dot kwak at gmail dot com
<?php function message($m)
{
echo "$m <br />\r";
return true;
}
$k=false;
if (message("first")&& $k && message("second")){;}
// will show
//first
class
$k=true;
if (message("first")&& $k && message("second")){;}
// will show
//first
//second
?>
$x = 1; $y = 2; if ($x != $y) $x = $x * $y; else $x = $x + $y; //can you guess the answer? echo $x; #2
It can be tricky to know what commands are executed if these expressions are stacked, here is an overview, general rule is that only the first statement following a true conditional will be executed. php > if (true) if (true) if (true) echo "1 "; echo "2 "; echo "3 "; echo "4 "; 1 2 3 4 php > if (true) if (true) if (false) echo "1 "; echo "2 "; echo "3 "; echo "4 "; 2 3 4 php > if (true) if (false) if (false) echo "1 "; echo "2 "; echo "3 "; echo "4 "; 2 3 4 if (false) if (false) if (false) echo "1 "; echo "2 "; echo "3 "; echo "4 "; 2 3 4
<?php
function b() {
echo 'b';
}
$a = true;
$a && b(); //b
$a = false;
$a && b(); //'IF' STATEMENT WRONG BEHAVIOR
If assign var to function that returns 0 as integer or 0 as string 'if' statement condition works as false.
Test script:
---------------
//In this script you can see, that set var value to 0 is equal to boolean false
//applied to if statement. There is no diff between integer 0 or string '0'
if($pos = 0){
$pos++;
} else {
$pos = 0;
}
var_dump($pos);
//$pos is not boolean false, so it should do if condition true, but it's not
//to make this work well i use this
$pos = 0;
if($pos !== false){
$pos++;
} else {
$pos = 0;
}
var_dump($pos);