str_replace() / str_ireplace() Since: PHP 4（2000）

// Replaces all matching occurrences.
str_replace($search, $replace, $subject, &$count);

// Replaces without distinguishing uppercase and lowercase.
str_ireplace($search, $replace, $subject, &$count);

Returns the string after replacement. If you pass a variable as the fourth argument, it will hold the number of replacements made.

<?php
// Basic string replacement.
$str = "apple and orange and apple";
echo str_replace("apple", "grape", $str); // Outputs: "grape and orange and grape"

// Pass arrays to replace multiple strings at once.
$template = "Dear {{Name}}, your order number is {{OrderNumber}}.";
$search = ["{{Name}}", "{{OrderNumber}}"];
$replace = ["John Smith", "A-12345"];
echo str_replace($search, $replace, $template); // Outputs: "Dear John Smith, your order number is A-12345."

// Use the fourth argument to get the replacement count.
$count = 0;
$result = str_replace("PHP", "Python", "PHP is fun. PHP is useful.", $count);
echo $result; // Outputs: "Python is fun. Python is useful."
echo $count; // Outputs: 2

// str_ireplace() is case-insensitive.
echo str_ireplace("hello", "Hi", "Hello HELLO hello"); // Outputs: "Hi Hi Hi"

// Replacing with an empty string removes the matched parts.
$html = "This is bold text.";
echo str_replace(["", ""], "", $html); // Outputs: "This is bold text."

// Normalizing line endings.
$text = "Line1\r\nLine2\rLine3\nLine4";
$unified = str_replace(["\r\n", "\r"], "\n", $text); // All line endings are unified to LF.

str_replace() replaces all occurrences of a specified substring within a string. By passing arrays for both the search and replace arguments, you can perform multiple replacements in one call. When using arrays, replacements are applied from left to right, so an earlier replacement may affect a subsequent one — keep this in mind.

str_ireplace() is the case-insensitive version. It is useful when processing strings that mix uppercase and lowercase, such as HTML tags or keywords.

For advanced replacements using regular expressions, use preg_replace(). To search within a string, use strpos(). To split a string, use explode().