How to add xml child to first index of an array using php

I have multiple arrays containing mathml(xml) elements.

For example my arraylist:

Array
(
    [0] => <mi>x</mi><mo>=</mo><mfrac><mrow><mo>-</mo><mi>b</mi>      <mo>±</mo><msqrt><msup><mi>b</mi><mn>2</mn></msup><mo>-</mo><mn>4</mn><mi>a</mi><mi>c</mi></msqrt></mrow><mrow><mn>2</mn><mi>a</mi></mrow></mfrac>
    [1] => <mi>x</mi><mo>+</mo><mn>2</mn><mo>=</mo><mn>3</mn>
    [2] => <mfrac><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>2</mn></mrow><mn>3</mn></mfrac><mo>-</mo><mn>3</mn><mo>=</mo><mn>2</mn>
    [3] => <mo>-</mo><mn>3</mn><mo>+</mo><mn>2</mn><mo>=</mo><mi>x</mi>
)

I want all array to start with <mo> but if it starts with <mo> then it's fine like array [3].

For example the expected output I want

Array
(
    [0] =><mo>+</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mo>-</mo><mi>b</mi><mo>±</mo><msqrt><msup><mi>b</mi><mn>2</mn></msup><mo>-</mo><mn>4</mn><mi>a</mi><mi>c</mi></msqrt></mrow><mrow><mn>2</mn><mi>a</mi></mrow></mfrac>
    [1] =><mo>+</mo><mi>x</mi><mo>+</mo><mn>2</mn><mo>=</mo><mn>3</mn>
    [2] =><mo>+</mo><mfrac><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>2</mn></mrow><mn>3</mn></mfrac><mo>-</mo><mn>3</mn><mo>=</mo><mn>2</mn>
    [3] =><mo>-</mo><mn>3</mn><mo>+</mo><mn>2</mn><mo>=</mo><mi>x</mi>
)

As you can see all array starts with <mo>.

Below is my current code to add it:

$ress = array();
$arr_result=[];
for ($i=0; $i <= $length ; $i++) { 
    $ress=$result[$i];
    if ($pos = (substr($ress,0,3)!="<mo>")) {
        $arr_result[]=array_unshift($ress, "<mo>+</mo>");
    }
    else{
        $arr_result[]=$ress;    
    }

}
print_r($arr_result);



$result store the array.

Solution

There are a lot if issues with the code you have, so I've just written something new...

$result = ["<mo>+</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mo>-</mo><mi>b</mi><mo>±</mo><msqrt><msup><mi>b</mi><mn>2</mn></msup><mo>-</mo><mn>4</mn><mi>a</mi><mi>c</mi></msqrt></mrow><mrow><mn>2</mn><mi>a</mi></mrow></mfrac>",
    "<mo>+</mo><mi>x</mi><mo>+</mo><mn>2</mn><mo>=</mo><mn>3</mn>",
    "<mfrac><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>2</mn></mrow><mn>3</mn></mfrac><mo>-</mo><mn>3</mn><mo>=</mo><mn>2</mn>",
    "<mo>-</mo><mn>3</mn><mo>+</mo><mn>2</mn><mo>=</mo><mi>x</mi>"
    ];
$arr_result=[];

for ($i=0; $i < count($result) ; $i++) {
    if (substr($result[$i],0,4)!="<mo>") {
        $arr_result[]= "<mo>+</mo>".$result[$i];
    }
    else    {
        $arr_result[]= $result[$i];
    }

}
print_r($arr_result);

This just goes through each line at a time, checking the first 4 chars for <mo> and adds them into the new value if it's not there.

Output is...

Array
(
    [0] => <mo>+</mo><mi>x</mi><mo>=</mo><mfrac><mrow><mo>-</mo><mi>b</mi><mo>±</mo><msqrt><msup><mi>b</mi><mn>2</mn></msup><mo>-</mo><mn>4</mn><mi>a</mi><mi>c</mi></msqrt></mrow><mrow><mn>2</mn><mi>a</mi></mrow></mfrac>
    [1] => <mo>+</mo><mi>x</mi><mo>+</mo><mn>2</mn><mo>=</mo><mn>3</mn>
    [2] => <mo>+</mo><mfrac><mrow><mn>2</mn><mi>x</mi><mo>+</mo><mn>2</mn></mrow><mn>3</mn></mfrac><mo>-</mo><mn>3</mn><mo>=</mo><mn>2</mn>
    [3] => <mo>-</mo><mn>3</mn><mo>+</mo><mn>2</mn><mo>=</mo><mi>x</mi>
)