Detailed explanation of PHP's built-in string processing functions

Characteristics of strings

1. Other types of data used in string type processing functions will automatically be converted into strings and processed

<?php
echo substr("abcdefghijklmn",2,4),"<br>";  //cdef
//使用数字会自动转化为字符串
echo substr(123456,2,4);  //3456
?>

2. Yes Treat strings as arrays and character sets

<?php
$str="abcdefg";
 
//下面这两种方法都可以输出想要的字符
echo $str[2]."<br>";
//但是为了区分数组我们常用下面一种
echo $str{2}."<br>";
?>

Powerful built-in string processing functions

1. Commonly used string output functions

echo()
print()
die()----exit()
printf() Format string
sprintf() Return formatted string

2. Commonly used String formatting function

Remove characters
ltrim(); Remove the string on the left (spaces are removed by default)
rtrim(); Remove the string on the right
trim(); Remove the strings on both sides

<?php
$str="  abc  ";
 
echo strlen($str)."<br>";
echo strlen(ltrim($str))."<br>";
echo strlen(rtrim($str))."<br>";
echo strlen(trim($str))."<br>";
 
$str1="123This is Test";
 
//第二个参数指定要删除的字符（串）
echo ltrim($str1,&#39;1&#39;)."<br>";
//删除所有的数字  0..9表示范围
echo ltrim($str1,&#39;0..9&#39;)."<br>";
?>

Add string

str_pad(); Add string (added on the right by default)

<?php
$str="hello";
 
//默认从右边补充
echo str_pad($str,10,"@")."<br>";
//两边补充
echo str_pad($str,10,"@",STR_PAD_BOTH)."<br>";
//从左边补充
echo str_pad($str,10,"@",STR_PAD_LEFT)."<br>";
?>

Case conversion

strtolower(); All characters are converted to lowercase
strtoupper(); All characters are converted to uppercase
ucfirst(); The first letter of each word is converted to uppercase
ucword(); The first letter of each word is converted Into uppercase

<?php
$str="My name is TOM!";
 
echo strtoupper($str)."<br>";
echo strtolower($str)."<br>";
echo ucfirst($str)."<br>";
echo ucwords($str)."<br>";
?>

String formatting related to HTML tags

nl2br(); The function inserts an HTML newline character (< before each new line (\n) in the string) br />).

htmlentities(); Function converts characters into HTML entities.

htmllspeciachars(); The function converts some predefined characters into HTML entities.

The predefined characters are:
& (ampersand) becomes &
"" (double quote) becomes "
'' (single quote) becomes '
< ( Less than) becomes< Add a backslash before the specified predefined characters:

<form>
  input:<input type="text" name="str" size="100">
     <input type="submit" name="sub" value="提交">
     <br>
</form>
 
 
<?php
echo $_GET["str"]."<br>";
//函数把字符转换为 HTML 实体。
echo htmlentities($_GET["str"],ENT_NOQUOTES)."<br>";
//函数把一些预定义的字符转换为 HTML 实体。
echo htmlspecialchars($_GET["str"])."<br>";
//去掉由addslashes()函数加的 \
echo stripslashes($_GET["str"])."<br>";
//结合使用
echo htmlentities(stripslashes($_GET["str"]))."<br>";
 
 
// 函数剥去 HTML、XML 以及 PHP 的标签。
echo strip_tags($_GET["str"])."<br>";
?>

<?php
$a=1000000.12345;
 
echo $a."<br>";
echo number_format($a)."<br>";
//小数点保留三位，千分位用“,”隔开，小数点用“.”
echo number_format($a,3,&#39;.&#39;,&#39;,&#39;)."<br>";
?>

<?php
$str="Hello World!";
 
echo $str."<br>";
echo strrev($str)."<br>";
?>

<?php
$pass="1234#!_56";
 
if(md5($pass)=="a8affa3d7aca3a35a39f674e1e5f0cc7")
  echo "Ok!";
?>

md5_file();

The function calculates the MD5 hash of the file.

The md5() function uses RSA data security, including the MD5 message digest algorithm.

<?php
$str1="abcd";
$str2="abcd";
 
if(strcmp($str1,$str2)==0){
  echo &#39;$str1=$str2&#39;;
}elseif(strcmp(str1,$str2)>0){
  echo &#39;$str1>$str2&#39;;
}else{
  echo &#39;$str1<$str2&#39;;
}
?>

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